Allow using of external plugins with environment variables

Change-Id: I4960fc1b7f263d875ff7b56fb26293d8da375a80
Reviewed-on: http://review.couchbase.org/26545
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>