OpenStack: update used limits tests.

Updates the get_limits request Mock and test so that it includes the
new totalFloatingIpsUsed limit.

Also, drops the totalKeyPairsUsed limit since it is no longer used
since https://bugs.launchpad.net/nova/+bug/1089877.
Merge pull request #1508 from dprince/openstack_limit_test_update

OpenStack: update used limits tests.