Update oslo.config to 1.4.0.0a2

oslo.config 1.4.0.0a1 has the config fixture which Keystone can
use.

1.4.0.0a2 has the config generator which Glance and Nova will
adopt, and cfgfilter which oslo.messaging wants to adopt.

Change-Id: I6c3049e6e379c83af7e6e7df8855b1a6c0b698e7
Merge "Update oslo.config to 1.4.0.0a2"