Get hosts without specifying segments

In current masakari API, operators should specify segment id to see hosts information.
But, there is a possibility that operators forget the relational of segments and hosts.
It would be hard to see hosts information in that case if there are multiple segments.

So this blueprint add the feature that gets host information without specifying segment id.
This feature will be implemented as new CLI command by combination of existed APIs.

This blueprint was discussed in below IRC meetings.
* http://eavesdrop.openstack.org/meetings/masakari/2017/masakari.2017-08-15-04.00.log.html
* http://eavesdrop.openstack.org/meetings/masakari/2017/masakari.2017-08-22-04.00.log.html