Drop @cached decorator from openstack_release()

The openstack_release() function is typically called (indirectly)
by os_release() which already does caching. The caching of
openstack_release() was preventing updates to the value during
upgrading to a new release of openstack.

Closes-Bug: #2037751
(cherry picked from commit b2d56ea)

charmhelpers/contrib/openstack/utils.py

@@ -581,7 +581,6 @@ def _do_install():
     return openstack_release().get('OPENSTACK_CODENAME')
 
 
-@cached
 def openstack_release():
     """Return /etc/os-release in a dict."""
     d = {}