Expose the early logging level for calico-node.
Use conf.node.FELIX_LOGSEVERITYSCREEN to set logging level in
BGPConfiguration and FelixConfiguration (whilst this is an odd
name/location it backwards compatible and will in most cases set
things as expected).
Change-Id: I70c3028423eddb4721456f645c4475da4af7ced5
3f79066797