Preserve exceptions in impl_zmq.

There is complication with re-raising exceptions and our usage of
eventlet.  If the code in the exception handler accesses the db or rpc
in the exception handler, it will no longer be able to re-raise the
exception.  Using excutils.save_and_reraise_exception() works aorund
this issue.

The most common error is calling LOG.error() or LOG.exception(), as it
is possible for these to go access rpc.  There is an option to turn on
notifications for these errors.

Fix bug 845866.

Change-Id: Ic914bba4703200ed114a42e3a55402883b430407
Merge "Preserve exceptions in impl_zmq."
Switch to oslo.config

The oslo-config package has been renamed. Switch to the new
version.

Change-Id: I6a0fa803b0076bc5338c996e21bbcef54e1d75cd
Update install_venv_common to use patch -N.

This patch makes it possible to call the post_process function
in install_venv_common (which currently only patches files)
multiple times.

The motivation here is that I would like to use this to patch
virtual env's created by tox. Tox doesn't provide a good hook
to call post_process (which patches files) only once so the
idea here is we'll just let it call post_process all the time...
and for distro's where patching is used to fix the venv the
patch -N option will make it skip patches on subsequent runs.

Change-Id: Ib0a36e49dd1884de131c569fe8bb66fe20285e97