import novaclient.v1_1.servers module direclty in nova driver

Actually, our patched nova client only support novaclient.v1_1, we need
to import novaclient.v1_1.servers in nova driver instead of using a
module from the patched client and that patch has been removed as
breaking the novaclient UT code.

Closes-Bug: #1363825
Change-Id: I66abd40952974e0eb422b95d1c6087c589957d93