test: Use non-default out/ dev/ paths in vstart

Currently, running "make check" nukes any existing vstart
setup in the tree.  This change makes it use test_dev/ and test_out/
so that it can peacefully coexist.

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #1374 from ceph/wip-vstart-paths

test: Use non-default out/ dev/ paths in vstart

Reviewed-by: Loic Dachary <loic@dachary.org>
Reviewed-by: Sage Weil <sage@inktank.com>