Promote nodes set with --set to sequence nodes as needed.

A node isn't known to be a sequence node until the YAML is parsed.
If a node sequence node was set on the command line, promote
it to a sequence node when it is discovered by YAML to be
a sequence node.

Fixes comment #18 in issue 921 (https://redmine.openinfosecfoundation.org/issues/921#change-4062)

Buildbot output:
https://buildbot.suricata-ids.org/builders/jasonish/builds/54
(pcap builder failed to start)