Every time the host certificates are renewed, there’s a chance to forget about
the copies of these for xrootd, which are per default in
“/etc/grid-security/xrd”.
Instead, the host certificate and key shall be read from the “standard”
locations.

• Set options to read the host certificate and key from
  “/etc/grid-security/hostcert.pem” respectively
  “/etc/grid-security/hostkey.pem”.

For this to work, the files must be readable by the user that runs the daemon,
which can, for example, be accomplished via ACLs using a command like:
setfacl -m u:xcache:r /etc/grid-security/hostkey.pem

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>

• Imported the systemd unit file for starting xcache within a singularity
  container.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>

• Imported cron jobs for updating the X.509 proxy certificate and the package
  repository data within the container.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>