Simplify the network sanity check using the new /etc/ci/mirror_info.sh
script. If thats present we use it to check pings to the mirror host and
HTTP against the pypi mirror. If it isn't present we don't bother. Note
this makes the sanity check fail the job again if it doesn't work which
should be fine at this point because /etc/ci/mirror_info.sh should only
exist if you have a mirror to provide information about.
Change-Id: Ifddb7c5b7ae3e446005338c9679dd5441bc57062
ce56c6704e