Add domain authorization scope

[bblrlo]

Add domain authorization scope as discribed in
http://developer.openstack.org/api-ref-identity-v3.html#authenticatePasswordScoped

[bblrlo]

json request must be like this:
{
"auth": {
"identity": {
"methods": [
"password"
],
"password": {
"user": {
"domain": {
"name": "domain-name"
},
"name": "usr",
"password": "pwd"
}
}
},
"scope": {
"project": {
"domain": {
"name": "domain-name"
},
"name": "prj-name"
}
}
}
}

[carolynvs]

Looks like this is specific to identity v3, which is on our roadmap but isn't implemented yet. When that is tackled, I'll make sure this is included.