``required_services`` validator should not check services which are
configured via ``api_versions@openstack`` context since the proper validation
is done at the context itself.
The inner check for ``api_versions@openstack`` in ``required_services``
checked only ``api_versions@openstack``, but ``api_versions`` string is also
valid name for the context (if there is no other ``api_versions`` contexts
for other platforms, but the case of name conflict is covered by another
check).
Also, this patch moves several imports inside validators to reduce cost
of discovering plugins.
Change-Id: I70af451d33ad8f11ab1ee0df0e18a93d045cf195
de10af53dc