glusterfs: Make commands that gluster calls work. Fixes #25620.

Done by setting PATH and PYTHONPATH appropriately.

Adds the following patches:

* One that removes hardcodes to /sbin, /usr/bin, etc.
  from gluster, so that programs like `lvm` and `xfs_info` can be
  called at runtime; see https://bugzilla.redhat.com/show_bug.cgi?id=1450546.
* One that fixes unsubstituted autoconf macros in paths (a problem
  in the 3.10 release); see https://bugzilla.redhat.com/show_bug.cgi?id=1450588.
* One that removes uses of the `find_library()` Python function that does
  not behave as expected in Python < 3.6 (and would not behave correctly
  even on 3.6 in nixpkgs due to #25763);
  see https://bugzilla.redhat.com/show_bug.cgi?id=1450593.

I think that all of these patches should be upstreamed.

Also adds tests to check that none of the Python based utilities
throw import errors, calling `--help` or equivalent on them.