Fix using non-default default_domain_id

Changing the default_domain_id to something other than the
default would cause several operations to not work correctly.
This is because the value of default_domain_id was read and
cached in several parts on import, which is before the call to
CONF() so the cached value is the original default ('default')
rather than the value that the operator has in the config file.

This change removes the cached default_domain_id and instead
reads the value when it's needed at runtime so that the
user-configured value is used instead of the original default.

Change-Id: I9543e2108866ed66000d75f868bbd2b1ed978b97
Closes-Bug: #1265108
Merge "Fix using non-default default_domain_id"