Provide API to disable specific providers

- The environment variables LCB_NO_HTTP and LCB_NO_CCCP may also be used
  to control these settings from the environment

Change-Id: I1b869510645b0b8854406f07d33542083d238b88
Reviewed-on: http://review.couchbase.org/33041
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>