(MCO-329) - Potential flaw in aes_security

In the past there existed a possible security flaw when enabling
learn_pubkeys on a MCollective server. Keys distributed in this manner
weren't verified at all, allowing for the possibility of a rogue key
being cached.

This commit increases the default level of security when using
learn_pubkeys. When enabling this configuration the client_public key
configured must be a X509 certificate while on the server side a CA cert
must be configured with the aes.ca_cert configuration option. The cert
will now be verified by the CA before writing it to disk.

The previous functionality can be restored by setting
aes.insecure_learning but be aware that enabling this should not be
configured in sensitive environments.

This commit also adds test coverage for Aes_security#decodemsg
MCO-373 Prepare changelog and releasenotes for 2.5.3
Merge pull request #5 from richardc/MCO-373_release_notes

MCO-373 Prepare changelog and releasenotes for 2.5.3
(RE-1497) Remove quantal from build_defaults

This commit removes quantal from all build defaults because it is end of
life. It removes the defaults from the build_defaults yaml.
Merge pull request #6 from melissaanne/MCO-329/stable/aes_security

(RE-1497) Remove quantal from build_defaults
Merge tag '2.5.3' into stable

2.5.3

* tag '2.5.3':
  (RE-1497) Remove quantal from build_defaults
  MCO-373 Prepare changelog and releasenotes for 2.5.3
  (MCO-329) - Potential flaw in aes_security
Merge branch 'stable'

Conflicts:
	ext/build_defaults.yaml