Add -c support to devtest_seed.sh.
Add -c support to devtest_seed.sh.

This is inline with the move away from undiscoverable environment
variables, but has no deeper purpose. USE_CACHE is still honoured in
devtest_seed.sh for backwards compatibility.

Change-Id: Id98ef0502ed4c6aa7fcfa2be107de65b6368d398

rdo-management/tripleo,rdo-management/tripleo