Fix StringIO import issue in svc-monitor

Commit 5930f33bc32743250d5a61c3b1f58ce239e053ad introduced
import of cStringIO.StringIO in svc-monitor, but stringIO
is redefined in another import.

Additionnaly, another couple of nits are fixed.

neutron_plugin_interface is also changed to set api_server_ip to
localhost if api-server.listen_ip_addr is 0.0.0.0 (the default)

Change-Id: I0608ace6953c77130beac26b7d48de72674fc6ba
Partial-Bug: #1335062
Merge "Fix StringIO import issue in svc-monitor"