Migrate to oslo.log

Migrate to oslo.log

It's mostly a matter of changing imports to a new location.

Non-obvious changes needed:
* pass overwrite= argument to oslo_context since oslo.log reads context
  from its thread local store and not local.store from incubator
* don't store context at local.store now that there is no code that
  would consume it
* LOG.deprecated() -> versionutils.report_deprecated_feature()
* dropped LOG.audit check from hacking rule since now the method does
  not exist
* WritableLogger is now located in oslo_log.loggers

Dropped log module from the tree. Also dropped local module that is now
of no use (and obsolete, as per oslo team).

Added versionutils back to openstack-common.conf since now we use the
module directly from neutron code and not just as a dependency of some
other oslo-incubator module.

Note: tempest tests are expected to be broken now, so instead of fixing
all the oslo.log related issues for the subtree in this patch, I only
added TODOs with directions for later fix.

Closes-Bug: #1425013
Change-Id: I310e059a815377579de6bb2aa204de168e72571e