SAML2 ECP auth plugin

Auth plugin authenticating against remote SAML based Identity Providers.
Upon successful authentication the plugin retrieves and stores unscoped token.
Plugin object should be later used for scoping the token.

Currently only HTTP BasicAuth Method is supported as
an IdP authn method.

Will not work for ADFS, as that has extensions to the document formats
as well as a different workflow.

Change-Id: Ieea40505a406bedf7219fa6f9e6cf29a45ad6e88
Implements: blueprint add-saml2-cli-authentication
Merge "SAML2 ECP auth plugin"