Add notifications for groups and roles

Currently, we only issue notifications for projects, users and
domains. This patch will add support to issue notifications for
groups and roles. Notifications will be sent on create, update
and delete.

I've also updated tests to include tests for projects and users.

DocImpact
Change-Id: I23eac9afb2a227e5739dc25395705597a92eb23c
fixes: bug 1231220
Merge "Add notifications for groups and roles"