DEFAULT_SERVICE_REGIONS is cumbersome when you have
more than one keystone endpoint and all you want to do
is set a global default service region.

This adds '*' as an optional fallback key to mean global default.
If an endpoint matches it will take precedence over the '*' value.

This also fixes the precedence order for DEFAULT_SERVICE_REGIONS
so that a user controlled cookie is used instead when that cookie
is valid for the given catalog. This changes the way the setting
works, but retains the intended result the setting was originally
intended for.

Change-Id: Ieefbd642d853fcfcf22a17d9edcc7daae72790a4
blueprint: global-default-service-region
Closes-Bug: #1772345
Related-Bugs: #1359774 #1703390