mtl_ofi_provider_include (resp. mtl_ofi_provider_exclude) can be used
to specify which provider(s) the OFI MTL can select (resp. ignore).
e.g. --mca mtl_ofi_provider_include "psm,sockets"
By default, mtl_ofi_provider_exclude is set to "sockets,mxm".
This deprecates the old MCA var named "mtl_ofi_provider".
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