[ofiwg] libfabric v1.8 schedule

[Hefty, Sean]

> There have been several fixes applied to the v1.7.x branch since the v1.7.1 release.
> To avoid too much overlap between the v1.7.2 and v1.8.0 release, I would like to start
> the v1.7.2 release cycle next Monday.

Thanks for everyone who cherry picked fixes for v1.7.x branch.  :)

For the v1.8 release, the target release date is June 28th.  Because of external schedule dependencies, I'm proposing we consider this a hard deadline.  There have been a significant number of changes since the v1.7.x branch was created.  To hit this, we would need to create RC1 within the next 2 weeks.

- Sean