Remove port bind pre-check

[BKreisel]

Removes port pre-bind check to address OS X permissions issue #680. This also removes the psutil dependency from the project.

Appropriate code to check for exceptions exists at bind time and from what I can see, removing the following lines will not introduce undefined behavior

[jmhodges]

Is the Popen executable="/bin/bash" change happening in another PR?

[BKreisel]

@jmhodges. Yea, i'm wrapping it up with all the other integration changes I did. I seperated this one out because it was more "controversial" 🎁

[kuba]

Does psutil not work in Mac OS X at all? What's the deal here? Could we fix it other way rather than removing totally? I don't see any alternatives discussed in #680.

I kind of liked @schoen's idea to check running processes and report back to the user which one is actually using the intended port ("hey, you have Apache running, please shut it down" is a big win for UX). NB It might not necessarily happen before bind - could do the check in exception handling phase. However, I do have to mention that the current implementation is not ideal either, c.f #255.

If you add/remove any dependency (psutil here) you have to update setup.py. You should also inform Debian devs to adjust their deps list (either cc: @fmarier, @hlieberman here, or report/submit PR at https://github.com/letsencrypt/letsencrypt-debian).

[BKreisel]

@kuba: Leaving psutil in just means that the integration tests on OS X are going to have to be executed with root privs.

The UX argument makes sense to me. I'm cool with leaving it in and closing this pull if that is the consensus.

[kuba]

How about we catch AccessDenied exceptions and ignore them silently (logger.info + inline comment about Mac incompat)! ;)

[kuba]

FTR the above idea is exploited in #886.