Add extra check to avoid double delete of instances

Add extra check to avoid double delete of instances

JIRA: FUNCTEST-748

Sometimes Openstack doesn't handle deletion in a timely manner which can lead
to openstack_clean to think there are leftovers from a previous test when in
fact the instances are pending deletion. This patch tries to better handle
this kind of situation that would otherwise result in a double free and
eventually <class 'nova.exception.InstanceInvalidState'> (HTTP 500)

Change-Id: Id0d9b61d8380e9b12fc7acd46cd84260714f4baf
Signed-off-by: Ciprian Barbu <ciprian.barbu@enea.com>