Split off from BZ 1576984.

Users may have already made settings changes at the server level for a number of servers.  With the newly added Region and Zone level Advanced Settings, it is now possible to make changes for all servers once at a higher level.  However, the way settings works is that more-specific changes take precendence over less-specific changes.  That is, if there is a change on a server, it will take precedence over being inherited from the zone or region.  This is by design.

However, users may want to set a value at the zone or region and "enforce" this new value down to all child resources, replacing any overrides they had put in place previously.

To do this, we will add a magic value into the advanced settings named <<enforce>>.
- If <<enforce>> is placed as a value at the zone level, then the existing value at the zone level will be forced into all child servers.
- If <<enforce>> is placed as a value at the region level, then the existing value at the region level will be forced into all child zones and their child servers.

Example:

Given,

at the Region:

  :api:
    :token_ttl: 1.minute

at the Zone:

  :api:
    :token_ttl: 5.minutes

at the Server:

  :api:
    :token_ttl: 10.minutes


If the user sets the Region to the following and hits save

  :api:
    :token_ttl: <<enforce>>

Then, the Region, and of its zones, and all of its servers will be set to

  :api:
    :token_ttl: 1.minute