The manager.py helper is launched with either py2 or py3 depending on
the version of the payload.  However, it was using openstack-origin
rather than actually picking up the actual version of the installed
software.  This patch ensures that the manager.py is run with the
python version of the installed payload, which resolves a bug when
upgrading to rocky (which is the py2 to py3 transition of the payload).

Also fixes an os.execl() bug which changed behaviour from python2 to
python3.

Change-Id: I17255b84ca067886f866a5f8311dcab57fba6f14
Closes-bug: #1818988

Major changes:

 * decoupling the hooks/manager.py file from the charm.  It is now a
   script that is called from hooks/keystone_utils.py as it has to use
   the same Python version/libraries as the installed keystone payload
   software.  keystone_utils.py and manager.py communicate via a Unix
   Domain Socket using json, encoded to base64.
 * As Python3 requires absolute imports, the charmhelpers symlink has
   been removed from hooks, and the hooks and charmhelpers symlinks have
   been removed from the actions directory.  Instead, the path is
   adjusted so that the modules can be found.

Change-Id: I18996e15d2d08b1dacf0533132eae880cbb9aa32