apiserver/admin: Restrict user logins

Any user login will fail unless that user exists in the environment.
Update blobstore dependency to pick up new checksum behaviour
Merge branch 'master' of https://github.com/juju/juju into login-environ-user-only
Merge pull request #712 from wallyworld/update-blobstore

Update blobstore dependency to pick up new checksum behaviour

Fixes: https://bugs.launchpad.net/bugs/1364750
Merge pull request #702 from waigani/login-environ-user-only

apiserver/admin: Restrict user logins

Any user login will fail unless that user exists in the environment.