Fixes create_saml_assertion() return

The create_saml_assertion() method only accepts project scoped tokens.
When using a domain scoped one it should return 403 correctly
informing the caller about this requirenment.

Change-Id: I1223f284a84dee05e4a3907e04345497e5f767c1
Closes-Bug: 1395117
Merge "Fixes create_saml_assertion() return"