Fix dangling patches in Cisco and Midonet tests

Cisco Nexus Tests:
Explicitly stops the patch to sys.modules immediately
after use to fix sporadic failures caused by the patch
not being handled correctly by mock.patch.stopall.

Midonet Interface Test:
Removes the double-patch of the 'device_exists' method
in ip_lib.

Closes-Bug: #1307025
Closes-Bug: #1307038
Change-Id: Ie22505bb8e406a61e40819d60c76d229916a6e01
Merge "Fix dangling patches in Cisco and Midonet tests"