dn2osdbk: handle internal references

The internal references were not properly created. This patches make the
:ref: inline markup create a <xref> tag when the reference is internal.

This patch also reworks the sphinx extension to handle the URLs that
will be generated in the hot-reference with the new :ref: behavior.

Closes-Bug: #1378073
Change-Id: I84bf268dc1055dc6b39f23c4415734184ec57eec
Merge "dn2osdbk: handle internal references"