Disable nova bm file injection in undercloud

We are now relying on host routes feature of neutron-dhcp-agent,
so it's time to disable nova bm file injection in undercloud
and use neutron-dhcp-agent instead (if file injection is enabled
the static route for metadata server is not set and overcloud
machines hit seed vm metadata server instead of undercloud one).

Co-Authored-By: Derek Higgins <derekh@redhat.com>

Partial-Bug: #1241042

Change-Id: I04a7fb0399031cbf475a427cceac080296f9bc8d
Merge "Disable nova bm file injection in undercloud"