Skip test early to improve memory footprint and time

When we skip the test class using skip_checks(), it check
the conditions and skip the test class at first step
without creating any keystone credentials. But when
tests are skipped with other decorator at test level then
it does create keystone credentials, setup network resources
and service clients.

Wehn all the tests in test class are skipped based on common
condition then it is better to skip them using the skip_check
so that we do not create any keystone, network resources which
will improve the DB queries keystone, neutron does and also
speed up the test skip.

Related-Bug: #2004780
Change-Id: Id5e6ddcb83aaa6133c28ef188183d98e26e4925b
14 files changed