DevStack variables Q_PLUGIN_EXTRA_CONF_{PATH|FILES} were deprecated
during the Ocata cycle. Instead, plugins should leverage the
neutron_server_config_add function. The networking-generic-switch
DevStack plugin was updated to use this function in commit
a656b25d037ad250647ae1484e75d9d4633ba373.
Using the deprecated variables is causing installation failures of the
DevStack configuration for Ironic multitenant networking. This patch
stops using them, which solves the installation issues.
Change-Id: If78640a46c3b8b2bfb5627de183a1fea45b777cf
Closes-Bug: #1715879
fe1bf7137f