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.
This is because the source tarball available on
https://download.gluster.org/pub/gluster/glusterfs/3.10/3.10.1/glusterfs-3.10.1.tar.gz
has different contents than the v3.10.1 tag;
for example, it lacks the file `xlators/features/ganesha/src/Makefile.am`,
which the tag has.
This is because GluserFS's release process removes some unused files.
This made impossible to apply patches written by or for upstream, as those
are written against what's in upstream's git.
As a nice side effect, we no longer have to hardcode the "3.10" in the
`3.10/${version}` part of the URL.
