Move default cache dir to persistent location

While this is called a "cache", it is important for it to survive. On
reboot, servers may need what was in the cfn config to restore complex
network configurations.

We introduce a new command line option, --backup-cachedir, that will
default to the old path, /var/run/os-collect-config. This will keep
things working for any tools that have been hard coded to use the old
path.

Change-Id: I78b3851b35addfc16913e3cd53c9d0e7eb3d191a