Remove auto-fallback to HTTP comms.

[hjoliver]

From @dpmatthews:

Python OpenSSL is required to use HTTPS. Otherwise cylc falls back to HTTP and issues lots of warnings (so it's not really very usable). I think we should:

• Allow [communication] method = http (only option at the moment is https) (assuming we want to support this).
• Add tests to make sure it works.
• Modify cylc check-software to test for OpenSSL (unless HTTP option configured).
• Get rid of the fall-back - i.e. fail if you've configured https method but can't use it.

Also:

• document the OpenSSL dependency

[hjoliver]

Current fallback is imperfect too. Without HTTPS/SSL support job submission to a remote host fails with SuiteServiceFileError: Couldn't get ssl.cert.

Which contradicts current documentation of "By default, the communication method is HTTPS secured with HTTP Digest Authentication. If the system does not support SSL, it will fall back to HTTP."

[chanwilson]

There's also a typo in the CUG in section 12.4; [communications] vs [communication]

[hjoliver]

There's also a typo in the CUG in section 12.4; [communications] vs [communication]

Fixed: 2e2ddd0