Add TLS encryption support for TSD connections

- Enable TLS encryption on TSD connections with command line
  toggle --tls or --ssl. Since OpenTSDB does not support SSL,
  this requires a SSL proxy in front of OpenTSDB, such as
  stunnel or similar

- Prefers TLS v1.2 if available (since python 2.7.9), uses
  TLS v1 otherwise

- Add _valid_certificate_name method to SenderThread, for
  verifying certificate name against hostname. Allows use of
  wildcard (*) in subdomains, but not in TLD or HOST parts.
  I.e. *.example.tld allowed

- Add command line option --ca-certs for specifying the path
  to the system ca-certificates file. Checks existence on
  start up. Defaults to /etc/ssl/certs/ca-certificates.crt

- Add EXTRA_ARGS option to init scripts, for specifying extra
  options like --tls and --ca-certs