rpcauth: Make it possible to provide a custom password

This adds the functionality to specify a custom password to
`rpcauth.py`, as well as makes the code (IMO) easier to understand.

share/rpcauth/README.md

@@ -8,3 +8,7 @@ Create login credentials for a JSON-RPC user.
 Usage:
 
     ./rpcauth.py <username>
+
+in which case the script will generate a password. To specify a custom password do:
+
+    ./rpcauth.py <username> <password>