Make Plugins more customizable

The keystoneclient plugin system is working really well, however i have come across some plugins that cannot be implemented with the specified plugin interface.

Primarily in auth_token middleware i want to write a plugin that controls the use of 'X-Subject-Token' and the message id on behalf of the service. There is no way for me to set any headers from the plugin other than X-Auth-Token.

Additionally I want to be able to authenticate always with a client certificate. If the client certificate is present there really is no need for the X-Auth-Token header, however the plugin must be able to access the connection parameters of the message. At the moment neither of these things is possible as a message without an X-Auth-Token is considered unauthenticated and the plugin cannot specify connection parameters.

Keeping with the existing plugin layout i propose to add 2 new functions to a plugin:
 - get_headers will replace get_token and will return a dictionary of things to add to the headers of a message. This will have to be done in a way that is still compatible with plugins that only implement get_token, however it will override get_token as the 'X-Auth-Token' header is just one element of the dictionary.

 - get_connection_params: will return a dictionary of connection parameters to be sent to requests.

No-Spec approval: http://eavesdrop.openstack.org/meetings/keystone/2015/keystone.2015-03-24-18.03.log.html