tox: Fix the tox.ini's to support older versions of tox

tox: Fix the tox.ini's to support older versions of tox

The src/cephadm/tox.ini and src/pybind/mgr/tox.ini both don't run
on older versions of tox.
When using tox 2.9.1 both fail for different reasons.

`src/cephadm/tox.ini` fails because `skipsdist=true` only works if it's
directly under the `[tox]` section.

`src/pybind/mgr/tox.ini` fails because older versions of tox can't find
the requirements.txt because they don't like whitespace between the `-r`
and `requirements.txt`.

This patch changes the tox.ini's to be backwards compatible for those
who happen to be running slightly older version of tox.

Signed-off-by: Matthew Oliver <moliver@suse.com>