Update configuration documentation for domain config
Update configuration documentation for domain config

The current configuration documentation describes the existing
file-based domain-specific configuration method. This patch
adds the information on the new SQL-based approach.

DocImpact

Partially Implements: blueprint domain-config-ext
Change-Id: Iff368cf05ab23f4c855ef4f1f286ff1081fd50ca

takeshineshiro/keystone,klmitch/keystone,jonnary/keystone,nuxeh/keystone,cernops/keystone,ajayaa/keystone,dstanek/keystone,klmitch/keystone,mahak/keystone,ilay09/keystone,cernops/keystone,dims/keystone,rajalokan/keystone,mahak/keystone,mahak/keystone,dstanek/keystone,jumpstarter-io/keystone,ilay09/keystone,openstack/keystone,ilay09/keystone,vivekdhayaal/keystone,jamielennox/keystone,openstack/keystone,takeshineshiro/keystone,jamielennox/keystone,openstack/keystone,jonnary/keystone,idjaw/keystone,maestro-hybrid-cloud/keystone,rajalokan/keystone,vivekdhayaal/keystone,rajalokan/keystone,maestro-hybrid-cloud/keystone,nuxeh/keystone,himanshu-setia/keystone,dims/keystone,roopali8/keystone,roopali8/keystone,vivekdhayaal/keystone,nuxeh/keystone,jumpstarter-io/keystone,jumpstarter-io/keystone,dstanek/keystone,ajayaa/keystone,himanshu-setia/keystone,idjaw/keystone