NetApp DHSS=True driver does not validate "netapp_root_volume_aggregate"

Description
===========
The DHSS=True driver for NetApp ONTAP storage system requires the configuration option netapp_root_volume_aggregate. If it isn't set, the driver fails to create a share server (and hence the share) at run time with an error traceback that's not exactly helpful: http://paste.openstack.org/show/803837/

It would be helpful to validate this parameter in the driver's setup method so that failures can be prevented at run time.

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

A chronological list of steps which will help reproduce the issue you hit:
* Use the NetApp driver with the following configuration

    [test]
    ...
    share_driver=manila.share.drivers.netapp.common.NetAppDriver
    driver_handles_share_servers=True
    netapp_login=<LOGIN>
    netapp_password=<PASSWORD>
    netapp_server_hostname=<NETAPP_HOSTNAME>
    share_backend_name=test
    netapp_transport_type=https
    netapp_storage_family=ontap_cluster
    netapp_server_port=443
    netapp_aggregate_name_search_pattern=(.*)
    netapp_root_volume_aggregate=
    netapp_root_volume_name=root
    ...
* Create a share network based on a VLAN network (NetApp doesn't support VXLAN or GENEVE)
* Create a share and check logs

Expected result
===============
Driver validates the "netapp_root_volume_aggregate" configuration prior so that a run time failure can be prevented. If the validation fails, perhaps the driver can fail to startup with an appropriate message - this forces an administrator to check and fix the issue at the right place. This is an opinion though.

Actual result
=============
Share was set to "error"

Environment
===========
1. OpenStack Train (reproducible in latest version as well)

2. NetApp ONTAP 9.x