MCO-486 expose SSL cipher to connector parameters

Here we add options to activemq and rabbitmq connectors to specify the desired
set of OpenSSL ciphers when communicating with the middleware.

As an example, this would choose ciphers from the TLSv1.2 suite, while
excluding some known weak ciphers.

     plugin.activemq.pool.1.ssl.ciphers = TLSv1.2:!MD5:!LOW:!EXPORT

See the OpenSSL documentation for an explanation of these cipher strings.
https://www.openssl.org/docs/apps/ciphers.html#CIPHER_STRINGS
Merge pull request #259 from richardc/MCO-486-expose_ciphers_to_connectors-stable

MCO-486 expose SSL cipher to connector parameters
MCO-502 update changelog and releasenotes for 2.6.1
Merge pull request #260 from richardc/MCO-502-changelog_2.6.1

MCO-502 update changelog and releasenotes for 2.6.1
MCO-511 Add documentation for ssl.ciphers connector options

Here we add documentation of the new
plugin.{rabbitmq,activemq}.pool.$n.ssl.ciphers options, added in
2.6.1.

As part of that we're adding a full parameter reference to the
reference/connector_activemq and reference/connector_rabbitmq pages
to make them more complete reference pages.
Merge pull request #261 from richardc/MCO-511-update_documentation_2.6.1

MCO-511 Add documentation for ssl.ciphers connector options
Merge branch 'stable'