made NS_SERVER_PLT environment variable override makefile default
made NS_SERVER_PLT environment variable override makefile default

This allows me to have NS_SERVER_PLT defined in .bashrc and preserve
it during git clean invocations.

Change-Id: Ia82f65c1df66dad3415811a0e2723e59780e8778
Reviewed-on: http://review.membase.org/4037
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Sean Lynch <9c3328a4a1f4f8f5c18b2d5b9057163c49b93ff3@literati.org>
Reviewed-by: Sean Lynch <9c3328a4a1f4f8f5c18b2d5b9057163c49b93ff3@literati.org>

ceejatec/ns_server,pavel-blagodov/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,vzasade/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,ceejatec/ns_server,membase/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,vzasade/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,daverigby/ns_server,ceejatec/ns_server,daverigby/ns_server,daverigby/ns_server,nimishzynga/ns_server,mschoch/ns_server,mschoch/ns_server,mschoch/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,mschoch/ns_server,nimishzynga/ns_server,vzasade/ns_server