[ostf] Default network error message improvement

[Damia Pastor]

While using Fuel Health-checks, namely:

- Create volume and attach it to instance
- Launch instance
- Check network connectivity from instance via floating IP
- Launch instance, create snapshot, launch instance from snapshot

Steps to reproduce:

- Rename or delete the network named 'net04' and create a new private network named x
- Run the next tests:
  > Create volume and attach it to instance
  > Launch instance
  > Check network connectivity from instance via floating IP
  > Launch instance, create snapshot, launch instance from snapshot

Real behaviour:

If there isn't a network named 'net04', it will fail and give this error message:

Private network was not created by default Please refer to OpenStack logs for more details

Expected behaviour:

There should be a more clear message mentioning 'net04' to help understand the requirement for this test.

I.e. Default private network 'net04' isn't present. Please verify it is properly created.

Links:
https://github.com/stackforge/fuel-ostf/blob/master/fuel_health/nmanager.py#L406

[Dima Shulyak (dshulyak)]

This is fix is not disruptive, so i recommend to fix it in 5.1.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/112560