Fix login deadlock
Merge pull request #381 from wallyworld/login-test-deadlock

Fix login deadlock

One of the login tests could deadlock. Change the test to start all N+1 attempts together, rather than N and then 1. 

Fixes lp:1347939
Merge branch 'master' of https://github.com/juju/juju into 150-environs-config-grow-a-uuid-method
Permit uuid to be set if unset, otherwise refuse to set it.