State remove setmongopassword

We create a user in the admin database
with read/write privileges to all databases,
so there's no need to add users to each
other database. Consequently, we can remove
the mongo.SetMongoPassword function and
change state.Machine.SetMongoPassword to
just call mongo.SetAdminMongoPassword.

Also removed unit.SetMongoPassword, which
isn't required anymore.

Tested live:
 - juju bootstrap && juju ensure-availability
 - juju upgrade-juju