calamari_rest: OSDList: Skip OSDs with no hostname

With a somewhat broken ceph cluster, with 2 OSDs, but only 1 up and in,
somehow I ended up with service_to_server having:

[[['b021bf32-9218-40ba-928c-ac98862ed738', 'osd', '0'], None],
 [['b021bf32-9218-40ba-928c-ac98862ed738', 'osd', '1'], 'ceph-0']]

This commit prevents a KeyError: None fault in this case.

Signed-off-by: Tim Serong <tserong@suse.com>