Fix detection of slave running for all mysql versions

[carlossscastro]

Fix detection of slave.running metric.

With this fix we retrieve the values of Slave_IO_Running and Slave_SQL_Running and if both are Yes slave.running is populated as 1 or 0 if any one of those 2 metrics is not Yes
The metric slave.running provided directly by mysql, using the logic above, was removed from version 5.7 and above.

[carlossscastro]

There were other approaches to obtain the status of the slave but as the integration already performs a show slave status query, and the information can be obtained from there, we did not add any extra query to keep the impact of the integration to the minimum.
More context on this or other solutions:
https://blog.pythian.com/verify-slave-running-mysql-5-7/

[roobre]

Looking nice overall! Left a couple questions and a few nitpicks.

[carlossscastro]

Fixes submitted