Detect missing packages with yum

yum -y doesn't report an error when packages are missing (see [1] for
upstream discussion).  Thus we run the output of yum through a small
awk script looking for missing packages and fail with an exit code
that gets picked up by -e and halts the script.

The one change required for RHEL is that python-wsgiref is included in
the distro python, so doesn't need a separate package.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=965567

Change-Id: I9908ff4edbf2b0d961d25837a08a34e1417bbb02