Set 'distro' when getting a host so that 'choose_init()' works on ubuntu

Otherwise hosts.debian.choose_init() will always return 'sysvinit' for
ubuntu, and we won't be able to create a custom-named cluster.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #219 from ceph/wip-hosts-choose-init

Set 'distro' when getting a host so that 'choose_init()' works on ubuntu