Updated cache module and its dependencies

This is to avoid cache module dependency on timeutils that are now moved
to oslo.utils.

The following changes are included:

 * neutron/openstack/common/cache/_backends/memory.py
  6ff6b4b Switch oslo-incubator to use oslo.utils and remove old modules
  2bedce3 Fix MemoryBackend not purging item from _keys_expired on delete

 * neutron/openstack/common/cache/backends.py
  39625e1 Set pbr 'warnerrors' option for doc build

 * neutron/openstack/common/cache/cache.py
  9c683be fix small typo

 * neutron/openstack/common/lockutils.py
  5d40e14 Remove code that moved to oslo.i18n
  7209975 Always log the releasing, even under failure
  bbb266c Clarify logging in lockutils
  942e1aa Use file locks by default again
  ac995be Fix E126 pep8 errors
  15b8352 Remove oslo.log from lockutils

Change-Id: I02cb4b2bc4b7bcba948e67cffdb8bd0219c89a29
Merge "Updated cache module and its dependencies"