This commit adds the --by-service flag in the discover_hosts
command of the nova cell subsystem, using this flag,
hosts are mapped based on the existing nova-compute services instead
of using the compute node reference.

In this way, we can ensure that the host mappings exists
for this new host (and create it if not) preventing the
HostMappingNotFound exception to loop over and over.

Bug: #1768094
Change-Id: I948004031bc3e125f929ed3e6c670aea11e73a31
Signed-off-by: Jorge Niedbalski <jorge.niedbalski@linaro.org>