keyring: make child of KeyStore

keyring: make child of KeyStore

This lets us pass a keyring to the auth methods as a source for keys for
doing the authentication handshaking.  Normally we pass a RotatatingKeyring
or the KeyServer, but for mon->mon we don't use a service key.  This will
let us use a simple KeyRing for that.

Signed-off-by: Sage Weil <sage@newdream.net>