Correct neutron-ns-metadata-proxy command when watch_log is False

Correct neutron-ns-metadata-proxy command when watch_log is False

Neutron[1] uses the option --metadata_proxy_watch_log=false to disable
log watch[2] in neutron-ns-metadata-proxy instances but should use the
option --nometadata_proxy_watch_log. It implies that
neutron-ns-metadata-proxy instances fail to start.

This changes updates neutron[1] to use the correct option.

The change also corrects associated functional tests[2], indeed
metadata_proxy_watch_log option has no effect if a log_file/dir is
defined for the agent running the neutron-ns-metadata-proxy.

[1] neutron.agent.common.config
[2] could be done by setting metadata_proxy_watch_log = false
[3] neutron.tests.functional.agent.test_l3_agent

Closes-Bug: #1490594
Change-Id: Iaec4a78847d802234c99514313440fd7c14bc554