Temporary solution for the PML inconsistency issue discussed in #7475.
This patch address 2 things: first it make the PMIx key optional so that
if we are not in a full modex mode we don't do a direct modex, and
second it get the PML info from the vpid 0 instead of from the local
rank.
Signed-off-by: George Bosilca <bosilca@icl.utk.edu>