Skip test_get_instance_action test for nova v3

InstanceActionsV3Test.test_get_instance_action() relies on the
x-compute-request-id header existing in the response from create_test_server()
in setUpClass(). That header name is going to be replaced for nova v3,
pending change I5e370fd3de5ee2f8a8d13553015d88910ff5ea87

So for now, skip that test in v3. The header name can be updated after the nova
change merges.

Change-Id: Ia1880dd53b7cbf34e66b349f01afb15672dffd12
Merge "Skip test_get_instance_action test for nova v3"