Allow encrypted and unencrypted connections on the same listener

A listener should allow encrypted and unencrypted connections, depending on configuration. The accepted values for the ssl-setting will be:

ssl=disable

ssl=allow

ssl=require

The old values such as "true" and "false" will still be supported, with their original meanings. The breaking change with this is that the REST-API output type of the listener and server ssl-parameter will be a string instead of a boolean.

Listeners will default to allow-mode, as it is the most flexible option and does not require additional configuration. Servers are still unencrypted by default, and do not (yet) support allow-mode.

Original description:
The Mariadb database listener allows SSL and unsecured connections on the same port.

Maxscale does not allow SSL and unsecured connections on the same port.

Documentation is clear on this:
https://mariadb.com/kb/en/mariadb-maxscale-2402-maxscale-2402-mariadb-maxscale-configuration-guide/#tlsssl-encryption
which states:
"Note: MaxScale does not allow mixed use of TLS/SSL and normal connections on the same port."

There should be a way allow mixed use so that the listener can match what the database offers. Example

SSL=TRUE

SSL=FALSE

SSL=MIXED