set sensible default values to ease configuration

signing key-pair settings
master_sign_key_name - name of the signing key-pair, without suffix
master_sign_pubkey - enable/disable signing the public key

signing signature settings
master_pubkey_signature - the pre-computed signature of master.pub + <sign_key>.pem
master_use_pubkey_signature - enable/disable usage of the pre-computed signature

verify_master_pub_sig - enable pubkey signature checking on the minion
switch between signing with own key-pair of usage of pre-computed signature only

if signing the pubkey is enabled, the master will switch between signing the pubkey
on the fly or just adding the pre-computed pubkeys signature to the reply.
if not already present, auto-generate a signing key-pair or use pre-computed signature

if signing the pubkey is enabled, the master will either generate a new signing key-pair
on the fly (if none is found) or use the pre-computed pubkey-signature. if the pre-computed
signature is enabled but not present, the master will complain and exit.