Remove use of request info to identify user in AuthComponent::login().

In your controller you now have to directly call identify() to verify user credentials
passed in request against db record. login() now just takes a user array as param
and writes the data to sesssion.
Rename AuthComponent::login() to AuthComponent::setUser().

The new name better conveys what the method does and prevents the possibility
of someone misunderstanding that it actually authenticates the user.
Removed unneeded method call.
Merge pull request #3735 from ADmad/3.0-auth

Remove use of request info to identify user in AuthComponent::login().