Remove use of 'cls.enabled = False'

Various network scenario tests have been setting this attribute
when raising skip exceptions.  It appears to no longer be used for
anything and can be removed.

Change-Id: I368f9e08cba6372010b9fed47f7730160e9d60b1
Skip IPv6 scenarios for baremetal

These tests attempt to create two isolated tenant networks and
boot servers on each.  Baremetal relies on flat networking that
does not support network isolation, so these tests should be skipped
in that case.

Change-Id: Ia7383114c17e4d901c81b8e7b6b2822a7ec4f908