1038242: add anaconda.pid check before chroot

The yum plugin 'productid.py' has to detect if it is
running in anaconda or not, so that it will place the
productid file in the correct chroot.

Previusly, this just checked for existince of '/mnt/sysimage'
but that can exist outside of anaconda (or in the original
bug, left over by anaconda). So add a check that
'/var/run/anaconda.pid' exists as well.
Merge pull request #910 from candlepin/alikins/1038242_yum_anaconda

1038242: add anaconda.pid check before chroot