gpush: add support for specifying multiple push sources

so far i assumed that --all would be enough, but now that i'm submitting
kernel patches in batches to avoid utterly flooding the list, being able
to explicitly specify multiple series seems quite useful.

Change-Id: I119745f95b079de6d1cbfed84184641ffcbc29c2
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>