Increase memory for Mothur tools

[jennaj]

Use case: https://biostar.usegalaxy.org/p/23685/#25087

[natefoo]

There are too many mothur tools to do this easily and it would be a hit to our throughput if we just did it for all of them. However, we should at least consider that a fair number of them use $GALAXY_SLOTS (and currently all of them are configured to run with only 1 slot). Ultimately I think there are two things that need to be done:

1. Convert our daily memory limit report to a graph in stats
2. Determine how much memory tools actually need with galaxyproject/galaxy#4862

[natefoo]

As of 2018-06-01 we are now collecting max memory usage with the cgroup metric plugin. We should have enough data to start making some decisions about this, and other tools, within a few weeks.

@nekrut: I'd propose dropping this from the fixathon based on the reason above.