refactor --with-prrte configure option

[hppritcha]

As part of the Open MPI fork of PRRTe migration into the Open MPI source tree proper, the configury code supporting the --with-prrte configure option needs to be refactored. Currently this option allows for building Open MPI against an external install of the upstream PRRTe. As part of this process we no longer want to support this.
We do still want to support disabling the build of the embedded prrte code - currently in 3rd-party/pprte.

Also, the configure option should be renamed to something more obvious to a packagers/installer. The suggestion would be to rename the option to --with-rte. The default would be yes, in which case the embedded prrte code would be built. The --with-rte=no should also be supported. But any other option to --with-rte should result in an erroring out at the configure stage.

The code that needs to refactored is in the <path-to-ompi-workarea>/config/ompi_setup_prrte.m4.

Changing the configure option from --with--prrte to --with-rte will be somewhat complicated and may be made easier by moving the location of the PRRTE git submodule to 3rd-party/rte.

Staging changes in as one PR solely to reject a ---with-prrte=<path_to_an_external_prrte> would be pretty straightforward. Changing the configure argument to --with-rte can be done in a subsequent PR.

Solving this issue will give one experience with coding m4.