Rally tests may leave networks on cloud

On a site where cs:openstack-service-checks-26 is installed, we are seeing rally networks not always being deleted.

Rally tests need to clean themselves up completely or stale rally networks/instances need to be reported by the charm for cleanup.

Here's an example rally network from the charm:

# openstack network show b8e62d31-727e-4d5b-8e47-b31b1a9e8d97
+---------------------------+---------------------------------------------------------------------------------------------------------------------------------------------+
| Field | Value |
+---------------------------+---------------------------------------------------------------------------------------------------------------------------------------------+
| admin_state_up | UP |
| availability_zone_hints | |
| availability_zones | nova |
| created_at | 2020-03-28T17:45:13Z |
| description | |
| dns_domain | |
| id | b8e62d31-727e-4d5b-8e47-b31b1a9e8d97 |
| ipv4_address_scope | None |
| ipv6_address_scope | None |
| is_default | None |
| is_vlan_transparent | None |
| location | cloud='', project.domain_id=, project.domain_name=, project.id='bc2d95bfe1a4469ab9b6e6fe16fab71b', project.name=, region_name='DCS1', zone= |
| mtu | 8950 |
| name | rally_verify_53ba7ff6_WZppbOTR |
| port_security_enabled | True |
| project_id | bc2d95bfe1a4469ab9b6e6fe16fab71b |
| provider:network_type | vxlan |
| provider:physical_network | None |
| provider:segmentation_id | 1052 |
| qos_policy_id | None |
| revision_number | 3 |
| router:external | Internal |
| segments | None |
| shared | True |
| status | ACTIVE |
| subnets | d07b0438-e155-4e11-9313-e043b51067bf |
| tags | |
| updated_at | 2020-03-28T17:45:19Z |
+---------------------------+---------------------------------------------------------------------------------------------------------------------------------------------+