This commit also introduces a dochelper concept. This provides
some convenience methods to the document pkg that help extract data
from known document types as well as walk document relationships to
discover related information such as BMC credentials for baremetal
hosts.
Once merged, a follow up patchset will leverage these within the
cloud-init code to deduplicate some of these lookups.
Change-Id: Ie6a770ce4b34adbea30281917f0cb2fdc460b4fb
e2cca32748