Fix random OverflowError exceptions in gate-neutron-python26 tests

These errors seem to be linked to the NEC plugin and http connections
not being stubbed out correctly. This patch ensures do_request is
stubbed out prior to the tests execution; this should be enough
to prevent timeouts from occurring again. Stubbing out the call has
no effect on the coverage of the agent scheduler tests for the NEC
plugin.

Fixes bug #1201478

Change-Id: Iee3cb3cb3137cedcf97c707ea7fde9114b7264b6
Merge "Fix random OverflowError exceptions in gate-neutron-python26 tests"