Refactor http/s handler & payloads

This commit moves much of the platform-specific logic from the
reverse_http handler down into the payloads. This makes the handler
a bit more agnostic of what the payload is (which is a good thing).
There is more to do here though, and things can be improved.

Handling of datastore settings has been changed to make room for the
ability to override the datastore completely when generating the
payloads. If a datastore is given via the `opts` then this is used
instead otherwise it falls back to the settings specified in the usual
datatstore location.

Down the track, we'll have a payload that supports multiple stages, and
the datastore will be generated on the fly, along with the stage itself.
Without this work, there's no other nice way of getting datastore
settings to be contained per-stager.

modules/payloads/singles/python/meterpreter_reverse_https.rb

@@ -36,7 +36,7 @@ module MetasploitModule
     opts[:scheme] = 'https'
     opts[:uri_uuid_mode] = :init_connect
     met = stage_meterpreter({
-      http_url:        generate_callback_url(opts),
+      url:             generate_callback_url(opts),
       http_user_agent: opts[:user_agent],
       http_proxy_host: opts[:proxy_host],
       http_proxy_port: opts[:proxy_port]