Suppress errors from markdown rendering

- A leading '<' breaks python-markdown in RHEL 6.4
  https://bugzilla.redhat.com/show_bug.cgi?id=1020170
- also adds an add_note method on the System model

Bug: 1020153
Change-Id: Ie47471c9887664f61faaa06d78c32c3d819fdd2e