Increase delay and retries for lxc cache download

This may increase run time, but thats better than having to re-run, esp
in the gate.

Related: #449
(cherry picked from commit 90f2b82c92789dcdf00fad51f657f5bb5cf4a4fd)
Merge pull request #468 from claco/proposed/juno

backport - proposed/juno - Increase delay and retries for lxc cache download