documenting that submit attributes are available also for BOSCO glideins
fixing usage message
6772: Set submit argument of -proxy to proxy_url value in factory

We need to pass in the proxy_url setting to the arguments for glidein
submission with the -proxy argument. (It needs to be a submit
argument because the bootstrap script should use it to pull down
everything.) This has been busted since v3.0.
Merge branch 'v3/6772' into branch_v3_2