Fix client usage in validation resources

Validation resources implicitly depends on tempest/clients.py as
it uses client aliases defined there. This is not ok since that
module does not live under tempest/lib.

Change validation resources to use clients as defined in
tempest/lib/services/clients.py instead. The ServiceClients class
defined in there is the superclass of the Manager class that was
assumed before, so it will just work with any further change.

Clients were also not used consistently with the use_neutron
flag, so making sure that only neutron clients are used if neutron
is selected and viceversa.

Change-Id: If83610b06af13aa736b03d463a94ac6ed02296bd
4 files changed