[r=axwalk],[bug=1275806] environs: deduplicate boilerplate config
Provider aliases (of which there is one: null->manual)
cause duplicated boilerplate config. We now remember
which boilerplate config templates have been seen, and
deduplicate.
Drive by addition of test to provider/manual for alias.
Fixes lp:1275806
https://codereview.appspot.com/60080045/