enabling adding external vitrage notifier

Currently any notifier in vitrage is imported statically. So developer can't add external notifier which is not pushed into open stack vitrage.
In order to support such external notifier the notifier shall imported in similar mechanism like datsources.
First of all in vitrage.conf would be conf under DEFAULT section named notifiers_path with all possible paths to notifiers packages.
Also there is conf with list of enabled notifiers called notifier.
For each notifier shall be a package with the same name, under one of the paths. In the __init__.py of the package shall be conf named notifier while path to the notifier class:
OPTS = [
    cfg.StrOpt('notifier',
               default='vitrage.notifier.plugins.aodh.'
                       'aodh_notifier.AodhNotifier',
               help=`the help`
               required=True),
]