Allow inheriting from multiple environments

[josh-bitovi]

There are times when it's useful to have multiple (configurable) levels of inheritance between environments. For example if you want a staging environment that inherits from default (as per usual) but also from prod, because it needs to be as close to prod as possible, with a few important tweaks.

Suggested config format:

    # in bitops helm config
    alsoInheritFrom: # will need to document this
      # default controlled by other env vars
      - staging

Currently default is handled by a slew of vars that let you control which parts of the default env are copied... but I'm not aware of these being used (i.e. I think we generally always inherit from default).

IMO it'd be better to:

• Always inherit from default
• Unless you specify the above config, in which case you can opt-in to default, or even put it in a different order

Another option would be to always use default (possibly retaining the vars to control if/how much of it is used), and just use the list for additional envs on top of default.

[PhillypHenning]

Beyond V2 - This is a major feature that will be introduced within the next few releases