ceph-volume: drop pytest-xdist usage

This is causing issues in ceph-volume unit tests (Still some details
not very clear so I can't explain for sure why for now).
Let's run the tests serially for now until I figure out what's wrong.

Signed-off-by: Guillaume Abrioux <[email protected]>

Author: guits

src/ceph-volume/tox.ini

@@ -11,7 +11,7 @@ deps=
 allowlist_externals=
   ./tox_install_command.sh
 install_command=./tox_install_command.sh {opts} {packages}
-commands=py.test --numprocesses=auto -vv {posargs:ceph_volume/tests} --ignore=ceph_volume/tests/functional
+commands=py.test -vv {posargs:ceph_volume/tests} --ignore=ceph_volume/tests/functional
 
 [testenv:py3-flake8]
 deps=flake8