Fixes including default SDN env file in deploy cmd

Fixes including default SDN env file in deploy cmd

The latest method to deploy opendaylight features upstream is to include
the default SDN file and then add-on the feature env file which should
override any default settings in the previous SDN env file.  For
example for bgpvpn:

openstack overcloud deploy -e <tht_dir>/neutron-opendaylight.yaml -e \
<tht_dir>/neutron-bgpvpn-opendaylight.yaml

This change documents the recursive function used to find the SDN env
files and includes the default for the SDN controller.

JIRA: APEX-555

Change-Id: I4270932ca4fbc21ea54e965d9d6491424f4463e2
Signed-off-by: Tim Rozet <trozet@redhat.com>