MAR-164: Fix Prosody TLS configuration in xmpp_server role:

- Added warning to role reference documentation about what DNS names
need to be included in the subject alternative name of issued
certificate used for Prosody.
- Added crontab with script that validates the certificate on daily
basis.
- Updated tests to include the proxy.DOMAIN and conference.DOMAIN DNS
names in subject alternative name for generated test certificates.
- Added and updated tests that cover new functionality.
- Fixed the Prosody TLS configuration to have common parameters
specified in general section, and any kind of overrides (mainly the
ciphers) in more specific sections.
- Updated release notes.