Error mounting device on create instance (LXC)

Description
===========

After installing Openstack in Ubuntu 18.04 and using LXC as virtualization engine, the creation of an instance fails with a log in `nova-compute.log` reporting that:

 ```Instance failed to spawn: TypeError: mount() takes exactly 4 arguments (3 given)```

Extra analysis of the logs an code points towards this file of `python-nova` package:

 /usr/lib/python2.7/dist-packages/nova/virt/disk/mount/api.py

The problem seems that the call at line 251

```
   246 def mnt_dev(self):
   247 """Mount the device into the file system."""
   248 LOG.debug("Mount %(dev)s on %(dir)s",
   249 {'dev': self.mapped_device, 'dir': self.mount_dir})
   250 out, err = nova.privsep.fs.mount(None, self.mapped_device,
   251 self.mount_dir)
   252 if err:
   253 self.error = _('Failed to mount filesystem: %s') % err
   254 LOG.debug(self.error)
   255 return False
   256
   257 self.mounted = True
   258 return True
```

is not matching the function signature defined in /usr/lib/python2.7/dist-packages/nova/privsep/fs.py as:

```
    30 def mount(fstype, device, mountpoint, options):
    31 mount_cmd = ['mount']
    32 if fstype:
    33 mount_cmd.extend(['-t', fstype])
    34 if options is not None:
    35 mount_cmd.extend(options)
    36 mount_cmd.extend([device, mountpoint])
    37 return processutils.execute(*mount_cmd)
```

So, it should be safe just to add `None` as `options` parameter.

Steps to reproduce
==================

Environment info:

  OS: Ubuntu 18.04.2 LTS
  Kernel: Linux 4.15.0-46-generic #49-Ubuntu SMP x86_64
  Openstack: queens

  Related packages versions:

  nova-api: Installed: 2:17.0.9-0ubuntu1
  python-nova: Installed: 2:17.0.9-0ubuntu1
  nova-compute: Installed: 2:17.0.9-0ubuntu1
  nova-compute-lxc: Installed: 2:17.0.9-0ubuntu1

Command executed to create the instance:

```
# openstack server create --flavor lxc.small --image lxc_ubuntu_18.04 --nic net-id=${NEUTRON_NETWORK_NAT_ID} --security-group default --key-name vagrant test_lxc_instance
+-------------------------------------+---------------------------------------------------------+
| Field | Value |
+-------------------------------------+---------------------------------------------------------+
| OS-DCF:diskConfig | MANUAL |
| OS-EXT-AZ:availability_zone | |
| OS-EXT-SRV-ATTR:host | None |
| OS-EXT-SRV-ATTR:hypervisor_hostname | None |
| OS-EXT-SRV-ATTR:instance_name | |
| OS-EXT-STS:power_state | NOSTATE |
| OS-EXT-STS:task_state | scheduling |
| OS-EXT-STS:vm_state | building |
| OS-SRV-USG:launched_at | None |
| OS-SRV-USG:terminated_at | None |
| accessIPv4 | |
| accessIPv6 | |
| addresses | |
| adminPass | GhYewcoXch3w |
| config_drive | |
| created | 2019-05-17T10:00:57Z |
| flavor | lxc.small (0) |
| hostId | |
| id | fe69997d-10c2-4850-bdba-2468da7148d6 |
| image | lxc_ubuntu_18.04 (de3ca120-42f6-47a2-be1b-65c7276f1566) |
| key_name | vagrant |
| name | test_lxc_instance |
| progress | 0 |
| project_id | 4c2cf8fa37c8451d87ddd2cf28cfd6bc |
| properties | |
| security_groups | name='35a8788c-364b-4da3-9c26-4aebf0ac04c4' |
| status | BUILD |
| updated | 2019-05-17T10:00:57Z |
| user_id | 0d48615bb8c747f4b5ec5bfc693b6832 |
| volumes_attached | |
+-------------------------------------+---------------------------------------------------------+
```

Expected result
===============
```
# openstack server list
+--------------------------------------+-------------------+--------+-----------------+------------------+-----------+
| ID | Name | Status | Networks | Image | Flavor |
+--------------------------------------+-------------------+--------+-----------------+------------------+-----------+
| dac7b2b8-841b-4a16-a54f-2d5177f05934 | test_lxc_instance | ACTIVE | nat=10.123.1.11 | lxc_ubuntu_18.04 | lxc.small |
+--------------------------------------+-------------------+--------+-----------------+------------------+-----------+
```

Actual result
=============
```
# openstack server list
+--------------------------------------+-------------------+--------+----------+------------------+-----------+
| ID | Name | Status | Networks | Image | Flavor |
+--------------------------------------+-------------------+--------+----------+------------------+-----------+
| fe69997d-10c2-4850-bdba-2468da7148d6 | test_lxc_instance | ERROR | | lxc_ubuntu_18.04 | lxc.small |
+--------------------------------------+-------------------+--------+----------+------------------+-----------+
```

Logs
====

Included some logs generated during the command, and retrieved using:

```
grep -r "2019-05-17 10:0[01]" /var/log >> openstack_queens_lxc_instance_creation_mount_error.logs
```