keyring: make it possible to create any arbitrary keyring with python-ceph-cfg

Currently only keyrings which have a "keyring_type" can be saved and auth_added.
It would be nice if it is possible to save/auth_add a keyring by defining a name,filename(if saving if necessary) and the key itself.
This could be used for defining keyrings for openstack, opennebula, libvirt, your own app...

Also a caller must be implemented then in sesceph and Cli (which is in the works? ;) )