Docs imply the mysql_variables is used to operate a single variable therefore
- fail before making any db connections if variable is not set
- validate chars for mysql variable name with re.match(^[a-z0-9_]+)
- use "SHOW VARIABLE WHERE Variable_name" instead of LIKE search
- getvariable() returns only the value or None if variable is not found
- the module returns only the found variable value instead of tuple for easier operation eg. as registere variable in tasks
