Related bug #1469434 fixed the usage comments for
Q_PLUGIN_EXTRA_CONF_FILES. However that change didn't
make it into neutron-legacy. This patch updates the comments
in neutron-legacy to reflect proper assignment of
Q_PLUGIN_EXTRA_CONF_FILES as well indicate
Q_PLUGIN_CONF_PATH is required when using extra conf files.
Change-Id: I447f1158d333ac4a35c4903a509146a62d93b272
Related-Bug: #1469434
Closes-Bug: #1542282