Update fabfile for two-server deployment.

Deployment is now done with two separate tasks: 'deploy' and 'migrate'.
'deploy' is used to update the git checkout and runs on both servers (rca2 &
rca3).  'migrate' runs database migrations and restarts celeryd and celerybeat.
This runs only on rca2, the primary server.

To deploy new code, do the following:

$ fab deploy migrate

If you're sure there are no new migrations, and celeryd/celerybeat don't need
to be changed, then you just do:

$ fab deploy