init null value in kairodb_writer

in tsinghua cluster, there is one node of the 90 nodes cluster that
doesn't contain fqdn attribute, so set hostname or ipaddress as the
host id of a node.

Cherry-pick from Change-Id: I2d8b5956954ca481932d89a0512cee54475e966e

Conflicts:

	chef/cookbooks/collectd/recipes/kairosdb.rb

Change-Id: I9b4910b01ae323a1b109a701b0fd69bc73bb56b3