Disable use of XRC in OpenIB BTL by default

Per discussions at the 7/25/17 devel core meeting, it was agreed that for now OpenIB XRC support will be disabled until issue #3890 is resolved.  Even though the issue is specific to the v2.x release (seemingly), the fact that no one is interested or has time to try and fix the issue, it was decided that XRC support will be disabled in master and the 3.0.x, 2.x, and 2.0.x releases.
