dgit: rpush: Use $isuite for $isuite, not $csuite

Send isuite as a parameter, and use it if it's provided, instead of
csuite.  This means config lookups like
    dgit-distro.SUITE.distro
only need the user-provided suite name (eg `unstable'), and don't need
to be duplicated for the canonical name (eg `sid').

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>