Ensure that session is rolled back on bulk creates

During bulk creates, the session is began explicitely;
ensure that it gets rolled back before re-raising in
order to avoid triggering InvalidRequestError
exceptions when the session is reused.

Partial-bug: 1244757

Change-Id: I797faeec2ca5374620db905599ab6e6f04fdcabd
(cherry picked from commit cb3f86dd781587eaa9fca6cc4a2f98de93526db7)
Set python hash seed to 0 in tox.ini

New tox (>=1.7.0) sets a random python hash seed by default. This is
generally good for testing because it will help keep projects working
regardless of the hash seed, but neutron unittests don't currently pass
with a random hash seed so set it to the python default seed.

This change will allow us to use new tox again and remove the
restriction on tox<=1.6.1 to run unittests.

Note this change will need to be backported to the stable branches to
keep unittests there working with new tox as well.

Change-Id: If151efeb8caae3111515191e238d6e6ae491cd44
Merge "Ensure that session is rolled back on bulk creates" into stable/havana
Merge "Set python hash seed to 0 in tox.ini" into stable/havana