Optional configuration for rockons

[ianlester7]

This change would allow optional configuration to be added to the json definition of a rockon (in rockon-registry).

Depending on the perceived importance of the option it could be displayed as top level configuration during installation or under an expandable advanced settings section.
Options should be pre-populated with default values or left blank in which case they will not be passed to the docker container.
It should also be possible to define a domain for options in the rockon config, these should be displayed as a dropdown (again with a default value or None)

Originally posted by @ianlester7 in rockstor/rockon-registry#270 (comment)

[FroggyFlox]

Adding a forum thread requesting this feature:
https://forum.rockstor.com/t/optional-environment-variables-in-rock-ons/8756.
This should be updated upon closure of this issue.