test: ceph-disk: run ceph-disk with cmake

test: ceph-disk: run ceph-disk with cmake

* CMakeLists.txt: do not run ceph-disk/run-tox.sh in its source
  directory anymore, "ceph" cli needs to run in the build directory.
* CMakeLists.txt, run-tox.sh:
  - pass $CEPH_BUILD_DIR down to tox.ini, so tox can set the working
    directory to $CEPH_BUILD_DIR to $CMAKE_BINARY_DIR, in which the
    "ceph" cli will be able to figure out where the pybind dso are
    located, and respawn itself with the necessary paths set accordingly.
  - pass $CEPH_BUILD_VIRTUALENV to tox, so ceph-helpers.sh is able to
    set the paths to ceph-disk and ceph-detect-init right.
* run-tox.sh: specify the full path of tox.ini, since we will call
  "tox" in the build directory.
* tox.ini:
  - move the non-flake8 related settings into [testenv:py27] section.
  - always use absolute path, as we will call "tox" in the build
    directory, instead where "tox.ini" lives.

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