Rightname confd's HMAC key

Currently, the ganeti-confd's HMAC key is called “cluster HMAC key” or
simply “HMAC key” everywhere. With the implementation of inter-cluster
instance moves, another HMAC key will be introduced for signing critical
data. They can not be the same, so this patch clarifies the purpose of the
“cluster HMAC key” by renaming it. The actual file name is not changed.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>