Some message sizes are not compatible with some tests

[blochl]

Now there is a default message size of 65536 for BW tests, and 2 for latency tests. This fails some tests, as for example, UC requires a message size not larger than the MTU.

I suggest having only one message size array (for cases when a user who knows what they do to be able to set manually) and for the 'usual' cases leave the default handling to perftest itself, which already has the knowledge which default is best for which test. ngc_multinode_perf does not need to have a duplication of this logic, and this way we avoid having 3 different arrays (at least!): for BW (non-UC), BW (UC), and lat tests.