Document testing plan for dependent tools

[gtkramer]

Prior to doing releases of tools on which the clr-distro-factory build process is dependent, those tools should be tested in this environment to identify any changes needed to the build process to support their release.

Given clr-distro-factory enables reproducible testing in namespaced environments on the same machine (or even running on a separate machine all together), we want to provide a test plan for these tools that if their updated versions allow for the build process to complete successfully, they may be released.

Given the integration with GitHub we have with clr-distro-factory now, I'm not sure if this is something that may be done as Travis checks on our dependent tools. I'm thinking the more automation, the better since we want to always make sure this is tested.

[gtkramer]

@otaviobp for thoughts from swupd
@mdhorn for thoughts from clr-installer
@reaganlo for thoughts from mixer