tomcat module doesn't allow to set mbean (or mbean domain)

@jsoriano We are looking into why the **tomcat** module is not working for us, and the issue is that the domain should be `Tomcat` instead of `Catalina`. I tried to override the mbean from its configuration file _tomcat.yml_, however it doesn't seem possible to set e.g. `cache.mbean` or `requests.mbean` ourselves.

_Originally posted by @dagwieers in https://github.com/elastic/beats/pull/13491_

Start with the tomcat module's tomcat.yml configuration and review the discussion in pull request #13491. Trace how cache.mbean and requests.mbean are handled, then make the configuration accept the requested MBean or domain override and verify that the Tomcat domain can be used.

Written by the indexing model from the issue text.