allow us to define tags inside config.taml

Tags are used to define on which runner a build can be executed, so people could add tags like: jdk8, winsdk8, msbuild, maven, ...

At this point tags can be added only from the user interface which makes harder to manage them when you add new runners.

We would like to be able to define tags in the config.taml file too (in addition to the ones from the user interface).

We need to be able to define different tags for each runner, but if you want you could also allow us to define tags per group, meaning that all the children will also inherit the tags from the group.