Fix #129 - wait for hosts to show up

It could also be made as a mini fixture dependent on 'setup_infrastructure_providers' but since we don't test host credentials apart from the refarch scenario (or did I miss something?), there doesn't seem to be a need for that atm.

https://github.com/RedHatQE/cfme_tests/blob/0502c3/fixtures/mgmt_system.py#L12