Only build a chroot if there isn't already one

Change-Id: I541f8424a30568ce2b7054d30f213e73bec99368
Raise an error if the chroot has already been created

This is almost a reversion of the previous commit, basically it will
allow us to do hivemind pbuilder.create || hivemind pbuilder.update in
the bash script of the builder which I think makes more sense.

Change-Id: Ib236495a76877d82a47e357e6cf33a0a72837452
Support repositories that have just debian branch

Change-Id: I9051fe220e23e630fcf0a3e1c96ad1aba198ea87