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
BSN: Remove db lock and add missing contexts

Adds context tracking decorators that were missing
from router interface methods. Without them, new
sessions were being created instead of using the
existing context which was causing transaction
issues.

Modifies the servermanager to store context references
as weakrefs so if multiple functions are called before
the rest functions are called, the first one doesn't steal
the only context reference with a pop() call.

Removes a DB lock for update in the server manager that occured
during rest calls that was triggering deadlocks due to the
file lock synchronization for the rest calls.

Closes-Bug: #1336251
Change-Id: Iad3d61e2c23832b3ad760a999fbab7feaa13f805
(cherry picked from commit ee9fe2458f813b5d367bc7263c7a30a3af46aa2b)
Merge "BSN: Remove db lock and add missing contexts" into stable/icehouse
Merge "Set python hash seed to 0 in tox.ini" into stable/icehouse