test: ceph-disk.sh: use ceph-helper.sh helpers

test: ceph-disk.sh: use ceph-helper.sh helpers

so we can use the utlitiy functions offered by ceph-helper.sh, and enable
cmake to run this test.

* ceph-disk/tests/ceph-disk.sh: it checks ../ for the executables for which
  fixtures are set up. but this assumption does not hold when we are using
  cmake for out-of-source build. so we need to pass the build directory to
  ceph-disk.sh. ${CEPH_BIN} is used for this purpose in this change, it is
  ".." by default, so this change is backward compatible. and one can run
  tox without setting up the ${CEPH_BIN} env variable.
* ceph-disk/test-requirements.txt: bump up tox to 2.0+
  in the case of "make check", both cmake and automake pass ${CEPH_BIN} env
  variables to the "make check" tests, so we can simpy pass it down via tox.ini's
  "passenv" setting. per
  https://testrun.org/tox/latest/example/basic.html#passing-down-environment-variables,
  this feature was introduce in tox 2.0. so bump up the minimum required
  tox version to 2.0 in ceph-disk/test-requirements.txt.
* ceph-disk/tox.ini: pass CEPH_BIN, CEPH_BIN, and CEPH_ROOT down to ceph-disk.sh

Signed-off-by: Kefu Chai <kchai@redhat.com>