Remove the skip for test_get_instance_action()

Change Ia1880dd53b7cbf34e66b349f01afb15672dffd12 introduced a skip
for InstanceActionsV3Test.test_get_instance_action(). This was due
to the header x-compute-request-id being changed to
x-openstack-request id. The name has since been updated (as nova
change I5e370fd3de5ee2f8a8d13553015d88910ff5ea87), so the skip
is no longer needed.

Also the skip in change Ia493d4838dee4a8a587d12a55f16afeb143d959d
is no longer needed.

Closes-Bug: 1281915
Change-Id: I4839ba9287f792c8ed352bb63391285a3afc0b84
Merge "Remove the skip for test_get_instance_action()"