Support webhook as notification method

Monasca should support notifications through Webhooks, as Ceilometer does with its Alarm Actions.
It would help Monasca's integration with Heat, specially the Autoscaling API.

The plan is to create a new Notification Type called "WEBHOOK", which will hold the "address" field. This address will be a URL address, which Monasca Notification will POST on this URL when the matching alarm triggers.
We also need to remove the constraint in Monasca-api, which currently only supports EMAIL.
Finally, the python-monascaclient and monasca-ui must support this new Notification type;