Allow environment for plugins on windows

Allow specifying the plugin via environment variables. This only works
for built-in plugins.

This should be replaced in the future with similar semantics for both
platforms

Change-Id: Ie8d3d27b5e14bacc8da87c6df185e55325cd6d34
Reviewed-on: http://review.couchbase.org/28006
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>