Make sure the NOVA_INSTANCES_PATH is created

Under some conditions when NOVA_STATE_PATH is set
the NOVA_INSTANCES_PATH directory under it is removed
but then not correctly recreated if the parent directory
is owned by root instead of the current user running
devstack. This change fixes that problem by creating
the NOVA_INSTANCES_PATH as root, and then chowning it
to the correct user.

Change-Id: I7fb724787b5e915bae9bd058454d2aa38991364e
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>