ADD description about MariaDB support and metrics semantics in "MySQL Checks"
ADD description about MariaDB support and metrics semantics in "MySQL Checks"

Add description about MariaDB support and metrics semantics in documentation of "MySQL Checks".

Change-Id: I50d849ac88a8a197cbaa5c75803d436be996719e
Closes-Bug: #1497172

sapcc/monasca-agent,sapcc/monasca-agent,sapcc/monasca-agent