support multiple serverwebroots in config.py

I'd like to make it possible to have multiple serverwebroots in config.py. serverwebroot is the string passed to rsync/ssh by fdroid server update to rsync the repo to a remote server. In our case, we run our repo on https://guardianproject.info/fdroid/repo on one server, and on an onion address on another server, so I want to have fdroid server update automatically update both.

I propose that serverwebroot should accept strings, lists, or tuples:

serverwebroot = 'foo@bar.com:/var/www/fdroid'

serverwebroot = ['foo@bar.com:/var/www/fdroid', 'bar@f.org:/fdroid']

serverwebroot = ('foo@bar.com:/var/www/fdroid', 'bar@f.org:/fdroid')