When Ironic used the 'cell_v2 discover_hosts' should be executed
with '--by-service' option, otherwise the hosts doesn't discoverd.
Change-Id: I213be691187b261523c2d3b1c51b2da8a50b412a
Signed-off-by: Maksim Malchuk <maksim.malchuk@gmail.com>
(cherry picked from commit 8af53d4f9b)