[TASK] build images for multiple repositories

[iamer]

the process definition needs to be adapted to build images for each supported repository.

Currently the imger participant expects to get all the info describing the image in a workitem dictionary (under 'image'). That aspect probably shouldn't change. Instead, the process definition should launch an imger with the right data for each repository

[iamer]

*** Bug 292936 has been marked as a duplicate of this bug. ***
Notes:

• remember that a repository can contain multiple architectures so there might be multiple image builds for one repository
• the workitem fields for excluding certain repos and archs by name should be honored. (See the build trial participants for code that does this.)