Use file locks by default again

After a lot of discussion[1], it was decided that this is the
safest way forward.  In the future we can investigate alternative
locking methods, probably as opt-ins so we don't break any
existing consumers of these locks.

[1] http://lists.openstack.org/pipermail/openstack-dev/2014-August/043090.html

Change-Id: I49ff98abc395d3263d55aeff26081b462a8a294e
Partial-Bug: 1327946
Merge "Use file locks by default again"