Read idp_metadata_path value from CONF.saml

Instead of CONF.federation.idp_metadata_path a
CONF.saml.idp_metadata_path should be used since it's a SAML specific
parameter.

Change-Id: I524b6a67abd110b13d8a5c38cdd9031eed771af3
Closes-Bug: #1372956
Merge "Read idp_metadata_path value from CONF.saml"