Template email messages for monasca-notification

Blueprint aim to extend monasca-notification component with templating email
messages.
Currently message is hard coded inside component.
Idea of this blueprint is to provide Jinja templates which should be loaded from
YAML file in specified configuration. User could provide his own templates with
email format which is supported in his company.

A result of his BP should be:
- create default template in YAML file
- add new option in config file which template should be loaded
- if specified template cannot be load/file is not exist use default template
- modify monasca-notification to load template and send message with new format