add env default option for rockons

[magicalyak]

This is related to rockstor/rockon-registry#174 where I realized we don't have a default option in the json that is respected for environment options in rockons.

1. adding default has not effect
2. I would like to be able to add default like the volume or port options for environment items

[FroggyFlox]

I really like this idea and would greatly improve the user-friendliness of the Rock-on install process, almost reducing it to a "Next > Next > Next..." experience. I found myself limited by that several times... That makes me ashamed not to have thought of implementing this in the same way as for the ports choice, as you suggest.

This shouldn't be too complicated to implement but would, however, require a slight addition to the database table DContainerEnv to store the default value set in the JSON file.
In order to keep compatibility with the existing Rock-ons, this would also need to be optional (which is better in my opinion anyway).

[FroggyFlox]

Adding a new feedback from the forum supporting the integration of this feature:
https://forum.rockstor.com/t/add-a-default-value-to-the-environment-object-of-rockon-conf-file/8633

That post should be updated upon resolution of this issue.

[FroggyFlox]

Adding another forum thread requesting this feature:
https://forum.rockstor.com/t/optional-environment-variables-in-rock-ons/8756