Add configuration to exclude full dst directories (#121)

[mikehw]

This consolidates free space evaluation into plot_util and adds a configuration point to not plot to a destination directory that is full.

[altendky]

So... This reads well to me. It definitely would have helped me this past week while I was plotting to misc drives I had laying around since my latest drive order got lost... but with our lack of tests I am hesitating a bit to just trust my reading of the code alone. If you want to take on the testing challenge as well, that would be a great help. If so, maybe start with another PR adding tests for the existing functionality that will act as regression checks for this PR. But, we haven't decided to mandate tests yet so I won't plan on requiring them here.

In case you don't have time to write tests, I'll call in reinforcements as well. @jkbecker and @chelseadole, want to take a look at this and help build some confidence here?

Also, sorry for the wait on this review. We do all appreciate the community contributions we are getting to include in plotman.

[mikehw]

I don’t think I’ll be able to work on unit tests until later next week. If someone else wants to hop in that would be great, otherwise I can add them when I get some time.

[altendky]

Not tonight, but I'll try to find some time to get tests in place for the existing code. Thanks again.

[robertwt7]

I was j ust searching for this.. Have 2 destination path but somehow plotman keep pointing to the one that is full

[Esaron]

I'd really like to see this merged. Given that's it's entirely opt-in and seems relatively trivial, I'd lean toward just resolving the conflicts and merging. Manual smoke testing to verify behavior is pretty straightforward.