implement forcing of enterprise edition via environment variable
implement forcing of enterprise edition via environment variable

I.e. for development and testing

Change-Id: I01137a90216a9a7c8b42cecd2717a314210c6975
Reviewed-on: http://review.couchbase.org/31494
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

ceejatec/ns_server,nimishzynga/ns_server,mschoch/ns_server,nimishzynga/ns_server,membase/ns_server,membase/ns_server,daverigby/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,membase/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,mschoch/ns_server,membase/ns_server,daverigby/ns_server,daverigby/ns_server,daverigby/ns_server,ceejatec/ns_server,nimishzynga/ns_server,mschoch/ns_server,mschoch/ns_server,mschoch/ns_server,ceejatec/ns_server,ceejatec/ns_server,daverigby/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,membase/ns_server