Add defaultBase for setting base for groups

[tripp]

@caridy @ericf
This pull request adds unit tests to #1720

[yahoocla]

CLA is valid!

[caridy]

+1

[tripp]

@caridy
In your initial pr, you'd mention that base is king and will always take precedence when set. If defaultBase is set, base will only take precedence if its set on the group. I think this is fine because this only arises if you explicitly set defaultBase and it can be overridden by explicitly setting base on the module. Setting base globally will not take precedence over defaultBase set globally. To have that happen, we'd probably have to set a flag.

[caridy]

I don't fully understand the question, here is what I know:

• if base is set, in config, or in a group, it is king.
• as today, base is NOT shared between different groups or core.
• if base is set at the module level, it is king.
• root is not shared or propagated between different groups or core.

what does this means?

• defaultBase is inherited in groups (from core)
• defaultBase (from group or core) + root (from group) defines base for group when needed (when no base is defined in group)
• defaultBase (from core) + root (from core) defines base for core when needed (when no base is defined in core)

[tripp]

That sounds like what we've got. I think I just misinterpreted you initial comments.