oo-accept-node: Simplify find_ext_net_dev

The node runtime simply defaults to eth0 if EXTERNAL_ETH_DEV is not set,
so make oo-accept-node do the same for consistency.
oo-accept-node: Use OpenShift::Config

Use OpenShift::Config to load and parse node.conf and resource_limits.conf
for consistency with the node runtime.

Use OpenShift::Config::NODE_CONF_FILE instead of defining NODE_CONF_FILE in
oo-accept-node.

Use the new globals $CONF and $RESOURCE_LIMITS instead of ENV for storing
the settings read from node.conf and resource_limits.conf.
Merge pull request #3335 from Miciah/oo-accept-node-use-Config-class

Merged by openshift-bot