agent: initialise mongo admin user before initialising state

This allows us to open mongo fully authenticated when opening
state, thus allowing the use of session.Copy.
Merge pull request #456 from axw/lp1351101-azure-mongo-bootstrap

agent: initialise mongo admin user before initialising state

This allows us to open mongo fully authenticated when opening
state, thus allowing the use of session.Copy.

Maybe fixes https://bugs.launchpad.net/juju-core/+bug/1351101