Previously the user update script only synced the SSH keys upon adding a user.
This will make it add new keys to gerrit on every run.
Fixes bug #1001180
Also added a lock file since it is sometimes taking longer than 15 minutes to
execute (especially now we are pulling SSH keys) and has jammed up StackForge.
Change-Id: I28fd0f557568c20a07e724bcd3d9c789a62fff42