Support nested domain-config elements

Nested domain-config inherit unset parameters from the domain-config
they are nested in. This helps avoid copy and pasted configs that are
almost the same except a few minor differences for a domain with
slightly different requirements.

For example: Consider a domain-config for example.com that, among other
settings, does not enforce hsts. Now if you want the rules for
example.com to apply to secure.example.com except that hsts _is_
enforced you can make a nested domain-config for secure.example.com
under example.com that sets hstsEnforced="true" and nothing else.

Change-Id: I9e33f7e62127fd7f4f15c3560fff2f2626477bd4