Fix runtime error under python 3.x

[zi0r]

The existing test is only compatible with python 2.x (which has been deprecated). Under python 3.x, you receive the following error:

% ./proxysql-nagios -d ~/test.cnf -H 127.0.0.1 -p 6032 -t rules --runtime
CRITICAL - ProxySQL check failed: Strings must be encoded before hashing

Calling encode() on the string before passing it to hashlib's md5() addresses this issue.