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.
Add new wrench points for upgrade-steps worker
Merge pull request #709 from mjs/upgrade-sync

cmd/jujud: synchronise state server upgrades

Use state's UpgradeInfo to synchronise state server upgrades. Overview:
    
- after restarting into a new tools version, master state servers will wait up to 15 minutes for any other state servers to indicate they are ready for upgrade. A downgrade to the previous tools version will occur if this wait times out.
- secondary state servers will wait until the master has completed its upgrade (to allow DB schema migrations to happen first)
- as state servers complete their upgrades they report this to the UpgradeInfo document. The document is archived once the last state server completes its upgrade.

Note that some refactoring to the implementation and tests is planned immediately after this PR lands. I didn't include them here because I didn't want to clutter up this PR any further.
cmd/jujud: avoid duplicate code for counting upgrade attempts in upgrade tests
cmd/jujud: avoid duplication in the 2 "success" upgrade test cases
cmd/jujud: shorten upgrade test helper names

Makes the tests a little more readable.
cmd/jujud: rationalise machine & config handling in upgrade tests

These tests have always been a bit confusing because machine-0 and its
config were being stored on the test suite, with various helper
methods magically using them as required. Data associated with other
machines have always been stored in local variables and passed around.

This change makes the data for machine-0 be handled consistently. This
makes for more explicit, clearer tests and has allowed for various
cleanups.