Tags instead of components

[Alexander Larsson <alexl redhat com>]

As a preparatory change for my work on using continuous for creating a
gnome platform/sdk I'd like to propose a change in how we pick what
components go in which target:

https://git.gnome.org/browse/gnome-continuous/commit/?h=wip/platform1.7&id=932cda02155d2c678c446e21c9b04dfd621ba023

Basically, instead of a "component" string property in each component we
have a list of tags, then the tags are used to put each component in one
or more targets. The advantage of this is that the multi-value lets us
avoid having a strict hierarchy of components that are used to construct
the targets. For instance, we can drop the artificial "minimal"
component and just list runtime *and* hwtest for the components in both
of these.

walters was a bit concerned that "tags" may be confused with "tag" which
defines what VCS tag to use. Maybe we can come up with a better name?