Correct network_device_mtu help string

This patch corrects the network_device_mtu help string. This setting
changes the mtu size for the interface not the vlan.

Change-Id: I2f3bcac42f08b70eb1649e3085c0de9af27b350c
Closes-bug: #1256696
Merge "Correct network_device_mtu help string"
Change conductor live migrate task to use select_destinations()

Live migration currently makes calls to scheduler select_hosts() to select
a suitable target host if none is given. Other tasks instead queries the
scheduler by making calls to select_destinations(). Since the latter call
basically returns the same result, and because live migration is the only
task using select_hosts(), changing to select_destionations() means the
scheduler would no longer need the select_hosts() call.

Change-Id: I984ccc2317184ba8f6b292c91717f52a81cf73d7
Deprecate scheduler select_hosts()

This call is no longer used and can be removed from the scheduler api.

Conductor manager, as the last user of this call, used it to select
a target for live migration, but now instead makes a call to
select_destinations() to get the same information. The related change
to conductor manager is here: https://review.openstack.org/#/c/52311/

Change-Id: I702515a086e7f33c48fa9ee106271c868b660540