Unittest builder for plugins fails if the minimum required platform version is unavailable
When plugin requires Pulp core version to be >= X.Y, builder tries to install version X.Y which may not exist for certain boxes.
OSTree plugin requires Pulp core to be >= 2.10, so builder tries to install Pulp 2.10 on F25 system but fails  because we do not have pulp-2.10 build for F25.
The PR builder job for plugins should be modified to:
1) Install the latest GA core RPMs and test against those. If they pass then pass the PR[UPDATE] This is already done; see the first associated commit.
2) If there are failures, retest using packages from the nightly build of Pulp. It's possible that there is new code that is required from platform that is not yet released. If this passes, then pass the PR