WIP: Client certificate support (#675)

[fdcds]

Adds the ssl_client_cert, ssl_client_key, ssl_client_key_password
and ssl_client_ca service parameters to the JIRA service, which allow
to connect to JIRA instances that are secured by SSL client certificates
issued by a private / self-signed CA.

The certificate key file may optionally be encrypted using a password,
which can be retrieved from an @oracle like regular passwords.

Closes: #675

WIP: Do not merge, yet.

.plan:

• Move parts of this into jira-python, where it probably belongs
  • Actually it should be moved even further down the stack, into requests, but that is still blocked by shortcomings of requests:
    • Password protected client certificate keys:
      • kennethreitz/requests#1573
      • kennethreitz/requests#2519
    • Server certificate verification against a custom CA, while using client certificates:
      • kennethreitz/requests#5103
  • Several issues exist regarding this topic in jira-python:
    • allow for cert based authentication pycontribs/jira#311
    • Client certificate not being passed pycontribs/jira#558
    • OAuth requests fails when server requires a client certificate pycontribs/jira#637
  • A PR exists to fix these issues:
    • Support client certificate authentication with OAuth and standalone pycontribs/jira#838
  • But it will not support password protected keys and a custom CA, since it relies on requests
• Cleanup
• Add end-user documentation

[ralphbean]

Any luck with this @fdcds?

[fdcds]

Any luck with this @fdcds?

The next slot for me to work on this will be Thursday night.

[ryneeverett]

triage: Any interest in rebasing and completing this?

[fdcds]

triage: Any interest in rebasing and completing this?

No, I do not use bugwarrior anymore.