Do not require method attribute on plugins

Removes the condition that an authentication plugin knows the "method"
name that is going to be used to call it. This condition prevents
different mechanisms like "kerberos" and "saml" from using the same
backend plugin.

The client should not know how the server is enforcing the Kerberos
authentication,  mod_auth_kerb or embedded Kerberos,  but the
mod_auth_kerb implementation needs to use the same implementation as an
X509 implementation.

Closes-Bug: #1343709

Change-Id: I6c7d44d3809e5e88cc50c50b6df6f3a154df7ab2
Merge "Do not require method attribute on plugins"