Add --[no]-collect-mysql

[fipar]

This PR adds the --collect-mysql option, which is enabled by default. If disabled (by specifying --no-collect-mysql) then none of the MySQL collectors (or checks, such as presence of mysql and mysqladmin on the path) are performed.

The goal is to make it possible to use pt-stalk with plugins that capture data about other systems (e.g. Cassandra) without having it failed due to missing mysql binaries on the host.