createToken.py: Allow specification of the JWT key file and output file

Current version of the script assumes that jwt key file is located in
the same directory as the script itself and token is written into the
same directory as json specification. This is not convinient for binary
distribution with key files placed under /etc and script under bin or
libexec dirs.
Allowing specification of key file and output files location fixes this
and application token can be generated with [1] while keeping backward
compatibility and existing behaviour (single argument to specify json
policy definition)

[1]
/usr/libexec/kuksa-createToken.py /usr/share/kuksa-val/jwt/all-read-write.json --key /etc/pki/kuksa/jwt.key --output /etc/xdg/AGL/%i/%i.token

kuksa_certificates/jwt/createToken.py

@@ -23,15 +23,16 @@
 from os import path
 
 
-def createJWTToken(input_filename, priv_key):
+def createJWTToken(input_filename, priv_key, output_filename=None):
     print("Reading JWT payload from {}".format(input_filename))
     with open(input_filename, "r") as file:
         payload = json.load(file)
 
     encoded = jwt.encode(payload, priv_key, algorithm="RS256")
-
-    output_filename = input_filename[:-5] if input_filename.endswith(".json") else input_filename
-    output_filename += ".token"
+
+    if output_filename is None:
+        output_filename = input_filename[:-5] if input_filename.endswith(".json") else input_filename
+        output_filename += ".token"
 
     print("Writing signed access token to {}".format(output_filename))
     with open(output_filename, "w") as output:
@@ -41,17 +42,19 @@ def createJWTToken(input_filename, priv_key):
 def main():
     parser = argparse.ArgumentParser()
     parser.add_argument("files", help="Read JWT payload from these files", nargs="+")
-    args = parser.parse_args()
-
     script_dir = path.abspath(path.dirname(__file__))
-    priv_key_filename = path.join(script_dir, "jwt.key")
+    default_key_filename = path.join(script_dir, "jwt.key")
+
+    parser.add_argument("--key", help="Private key location", dest="priv_key_filename", default=default_key_filename)
+    parser.add_argument("--output", help="Name of the output file to store token to", dest="output")
+    args = parser.parse_args()
 
     print("Reading private key from {}".format("jwt.key"))
-    with open(priv_key_filename, "r") as file:
+    with open(args.priv_key_filename, "r") as file:
         priv_key = file.read()
 
     for input in args.files:
-        createJWTToken(input, priv_key)
+        createJWTToken(input, priv_key, args.output)
 
 
 if __name__ == "__main__":