Currently Ceilometer uses redundant indexes in MongoDB and in same
time doesn't cover get_resources sorting by indexes.
In this CR source field indexing is deleted and resource.project_id,
resource.user_id indexes are combined to one.
New compound index covers the get_resource sorting and its parts
are used in queries by timestamp, project_id, user_id.
Change-Id: I077317533fd30865eea8a3f32e3c3a7f53fa6343
Closes: bug 1433924