SERVER-114 - add endpoint to flush env cache

ca209180e734c1883aae6ac42265f3737b02d172 was done as a separate commit in case it is decided that the API change is not desirable.

eee5c690243580c447bb539d7ee5c0394b6180cd contains a rather hilarious amount of change in the test code considering how much little functionality was added.  I'll add some comments there in an attempt to make the review easier.
I think I'm good with letting this be generic for more "unit" level testing and deferring to a smaller set of integration tests that bootstrap the webserver and hit the real API like you did in "puppet-admin-int-test".  Seems reasonable to me.