Code review comment for ~raharper/curtin:fix/jenkins-filter-once-more

Running a test-job with proposed enabled using three filters

target_distro=ubuntu
target_release=bionic
conf_file=*basic.yaml

Which selects test_basic.py:BionicTestBasic and runs it with -proposed pocket enabled.

If this is successful, then we can update the -proposed jenkins jobs to use the
target_distro=ubuntu filter to ensure we only run Ubuntu proposed upgrades (ignoring Centos)
And vmtest currently already knows to not test -proposed on the devel release.

https://jenkins.ubuntu.com/server/job/curtin-vmtest-devel-debug/109/console