Fix Message format-string parsing

The parsing in
openstack/common/gettextutils.Message._save_dictionary_parameter
does not account for two or more format directives in direct
succession, e.g.:

%(key1)s%(key2)s

causing key/value pairs to not be copied during a mod operation.

Fixes bug 1207789

Change-Id: I56940003de3e6e2fa7dd08604d04f4c57586b0a3
Merge "Fix Message format-string parsing"