config options to specify preferred key algorithms for zts sign operations

[havetisyan]

Three config options are setup with default EC value to maintain existing (and preferred) behavior:

If the ZTS has both EC and RSA private keys defined, then this setting
# specifies the preferred algorithm for signing json web objects such
# as access tokens, jws policy files, etc.
#athenz.zts.key_algo_json_web_objects=EC

# If the ZTS has both EC and RSA private keys defined, then this setting
# specifies the preferred algorithm for signing proprietary objects such
# role tokens, json policy files, etc.
#athenz.zts.key_algo_proprietary_objects=EC

# If the ZTS has both EC and RSA private keys defined, then this setting
# specifies the preferred algorithm for the private key to be passed
# to the plugins if they require the server's private key
#athenz.zts.key_algo_plugins=EC