make the opal progress yield variable settable at any time

The semantics of the variable mpi_yield_when_idle are to call
opal_progress_set_yield_when_idle at MPI_Init. It would be difficult
to modify the old variable to support setting this parameter at
runtime. The fix is to add an additional parameter to opal:
opal_progress_yield_when_idle that directly sets the variable. This
variable is settable anytime and does not affect the semantics of
the old mpi_yield_when_idle variable.

Refs #193

cmr=v1.8.1:reviewer=jsquyres