core: StoragePoolValidatorTest config mocking

Removed redundant mocking that's not used in the test and streamlined
the existing mocking to use two version - 3.0 as an "old" version which
does not support new features and 3.1 and a "new" version which does.

Bug-Url: https://bugzilla.redhat.com/1127590
Change-Id: If916e1080686ec037dba6f79264307e73b1502d2
Signed-off-by: Allon Mureinik <amureini@redhat.com>