Add flag to enable cross domain requests in Api

Add the -api-enable-cors flag when running docker
in daemon mode to allow CORS requests to be made to
the Remote Api.  The default value is false for this
flag to not allow cross origin request to be made.

Also added a handler for OPTIONS requests the standard
for cross domain requests is to initially make an
OPTIONS request to the api.
Add Access-Control-Allow-Methods header

Add the Access-Control-Allow-Methods header so that
DELETE operations are allowed.

Also move the write CORS headers method before
docker writes a 404 not found so that the client
receives the correct response and not an invalid
CORS request.
Add Cors and OPTIONS route unit tests

Move creating the router and populating the
routes to a separate function outside of
ListenAndServe to allow unit tests to make
assertions on the configured routes and handler
funcs.
Add explicit status response to OPTIONS handler

Write the http.StatusOK header in the OPTIONS
handler and update the unit tests to refer to the
response code using the const from the http package.
Add OPTIONS to route map

Move the OPTIONS method registration into the existing
route map.  Also add support for empty paths in
the map.
* Remote Api: Add flag to enable cross domain requests