Refactored headers produced by middleware (bug 835087)
Refactored headers produced by middleware (bug 835087)

- Renamed HTTP_X_ROLE to HTTP_X_ROLES (bug 835087)
- Simplified logic to build the list of roles
- Introduced HTTP_X_USER_ID and HTTP_X_USER_NAME headers
- Retained support for HTTP_X_USER and HTTP_X_ROLE, but
  documented them as *Deprecated*
- Documented all headers produced by middleware

Change-Id: I03fb45879ba3bf37d0bef1d0c110b44f55b7c10c

idjaw/keystone,JioCloud/keystone,derekchiang/keystone,jamielennox/keystone,rajalokan/keystone,dsiddharth/access-keys,cbrucks/Federated_Keystone,openstack/keystone,cernops/keystone,ntt-sic/keystone,ntt-sic/keystone,MaheshIBM/keystone,cbrucks/keystone_ldap,townbull/keystone-dtrust,cloudbau/keystone,reeshupatel/demo,vivekdhayaal/keystone,rodrigods/keystone,sileht/deb-openstack-keystone,reeshupatel/demo,citrix-openstack-build/keystone,dsiddharth/access-keys,sileht/deb-openstack-keystone,promptworks/keystone,MaheshIBM/keystone,mahak/keystone,mahak/keystone,ilay09/keystone,openstack/keystone,jumpstarter-io/keystone,sileht/deb-openstack-keystone,maestro-hybrid-cloud/keystone,cbrucks/keystone_ldap,nuxeh/keystone,cloudbau/keystone,vivekdhayaal/keystone,ntt-sic/keystone,rushiagr/keystone,rushiagr/keystone,kwss/keystone,nuxeh/keystone,roopali8/keystone,savi-dev/keystone,klmitch/keystone,maestro-hybrid-cloud/keystone,klmitch/keystone,citrix-openstack-build/keystone,UTSA-ICS/keystone-kerberos,blueboxgroup/keystone,jumpstarter-io/keystone,mahak/keystone,jonnary/keystone,dstanek/keystone,cbrucks/Federated_Keystone,JioCloud/keystone,ajayaa/keystone,ging/keystone,roopali8/keystone,admiyo/keystone,blueboxgroup/keystone,dims/keystone,ging/keystone,nuxeh/keystone,promptworks/keystone,cbrucks/keystone_ldap,idjaw/keystone,UTSA-ICS/keystone-kerberos,rodrigods/keystone,savi-dev/keystone,townbull/keystone-dtrust,rickerc/keystone_audit,derekchiang/keystone,himanshu-setia/keystone,openstack/keystone,derekchiang/keystone,promptworks/keystone,dims/keystone,dstanek/keystone,jonnary/keystone,townbull/keystone-dtrust,kwss/keystone,ilay09/keystone,admiyo/keystone,rajalokan/keystone,jumpstarter-io/keystone,citrix-openstack-build/keystone,takeshineshiro/keystone,cernops/keystone,rushiagr/keystone,jamielennox/keystone,savi-dev/keystone,kwss/keystone,cbrucks/Federated_Keystone,reeshupatel/demo,admiyo/keystone,rickerc/keystone_audit,dstanek/keystone,takeshineshiro/keystone,rajalokan/keystone,ajayaa/keystone,dsiddharth/access-keys,vivekdhayaal/keystone,cloudbau/keystone,ilay09/keystone,rickerc/keystone_audit,himanshu-setia/keystone