Add ejabberd_node_utils:file_exists/2 that takes a nodes into account

Previously there only a function that took a node name from from the CT
ejabberd_node config value.
Change certificate file location to /tmp/server.pem

The server.pem file is copied to /tmp while running devrel Make target.
Merge pull request #95 from esl/fail-test-with-auto-skipped-cases

Fix connect_SUITE