make available custom reports able to differ between domains for domains
that all map to the same submodule by specifying a report_map (e.g.
CUSTOM_REPORTS) as a dict of lists of reports, keyed by domain, instead of a simple
list of reports

Scenario:

HSPH has hsph, hsph-dev, hsph-betterbirth-pilot-2 domains.
hsph-dev and hsph-betterbirth-pilot-2 use the same reports, and they are
evolutions of those used earlier in hsph, but have diverged
substantially.  Rather than make a separate submodule for the new
reports, let's just allow them to be in the same subdomain because:

- potential for reuse of CouchDB and Python code is still high
- it takes a lot of effort to create a new submodule
Merge pull request #237 from dimagi/report-map

make available custom reports able to differ between domains