Update to pull down new tags as well.
Move mongo tasks to the mongo role.
Update playbooks to use the mongo role.
Update role prefix in names.
Merge pull request #351 from edx/feanil/mongo_role

Feanil/mongo role
Merge pull request #350 from edx/feanil/gh_mirror_tags

Update to pull down new tags as well.
bug fixes.
Merge pull request #352 from edx/e0d/fix-mongo-user

bug fixes.
Fixed bad YAML and locked db to specific variable configured version
Merge pull request #353 from edx/cg/mongodb_version_lock

Fixed bad YAML and locked db to specific variable configured version
updating from master, resolving merge conflicts