auth: make g_conf.keyring a plain old string

auth: make g_conf.keyring a plain old string

Make g_conf.keyring a plain old string rather than an array of strings.
Don't do substitution using the user's HOME variable-- this could lead
to security holes for setuid processes.

Get rid of AuthMonitor::read_keyfile because there is already a Keyring
member function, Keyring::load, that does the same thing.

qa/rbd/common.sh: we can now use cconf to figure out what the keyring
is.

Signed-off-by: Colin McCabe <colinm@hq.newdream.net>