Make a generic setting to configure certificate validation

[zoni]

XMPP currently has a XMPP_CA_CERT_FILE that can be set to adjust how certificate validation is done for XMPP back-ends.

It would be nice however if there would be a generic setting that configures it for any back-end that is being used, where this is applicable.

Note: We should always make sure the default is as secure and safe as possible. Make users lower security if they want, but keep it secure by default