Deployed dev copy of feature branches

[Bjwebb]

Not sure how best to write this up in a generic way, so we should document the Cove specific case first of all.

Concrete example:
OpenDataServices/cove#76
branch: minor-live-improvements
Added to bottom of pillar https://github.com/OpenDataServices/opendataservices-deploy/blob/master/pillar/dev_pillar.sls
Deployed using highstate at http://minor-live-improvements.dev.cove.opendataservices.coop/

(Internally we create a cove macro, and apply this for every time a branch happens.)