Extract functions for interacting with tested ejabberd node

These functions were implemented in conf_reload_SUITE but the
functionality is also required in the connect_SUITE.
Add information on how to use ejabberd_node_utils module to README
Test that a client cannot authenticate without TLS when it is required

The test reveals a bug that were allowing a client to authenticate
without TLS after requesting compressed connection - although TLS was
required by the server.
Make rebar use appropriate feature branch of escalus

NOTE: This commit is to be reverted when merging.