ganeti-masterd: avoid SimpleConfigReader

SimpleStore is a lot less heavyweight than SimpleConfigReader, and to
just get the master name we can use that. This is the only usage of
SimpleConfigReader currently, but we're not going to delete the class,
as new usages will come in for ganeti-confd (in 2.1). Using it there,
though, will make the class even more heavy to load, so it makes sense
for this simple usage to be converted.

Signed-off-by: Guido Trotter <ultrotter@google.com>