cephadm: fix autopep8 arg parsing on newer tox versions

cephadm: fix autopep8 arg parsing on newer tox versions

This was failing for me on new tox versions with

fix: commands[1]> autopep8 --max-line-length 100
incorrect number of argumentsfix: exit 99 (0.06 seconds) /home/adking/ceph/src/cephadm> autopep8 --max-line-length 100 pid=309039
fix: FAIL ✖ in 0.08 seconds

As it was considering our list of args for autopep8 to each be
one separate command. Fixed by just adding slashes to the ends
of the lines (as has already been done in the tox.ini in src/pybind/mgr
for some of the tests there)

Signed-off-by: Adam King <adking@redhat.com>