* sensu: drop Ubuntu support
Upstream removed Ubuntu repositories.
Train/Stein only
* Bifrost: pin IPA builder to 2.4.0
Train & earlier
IPA builder master branch now requires diskimage-builder 3.4.0, but
Python 2 is supported only up to DIB 2.38.
This change works around the issue by pinning IPA builder to 2.4.0,
which has a Python 2 compatible dependency on DIB.
Change-Id: Ice125e101a689fd3df9d1cd9a093a026f9747a3e