requirements.txt: Move the six dependency into the general requirements

requirements.txt: Move the six dependency into the general requirements

config_template.py depends on six, which isn't listed in the default
requirements.txt. This previously frequently wasn't a problem, because
six used to be a standard package being installed into a venv, and
lots of other projects depended on it.

It also does get installed for unit and integration tests via
tests/requirements.txt, so any broken dependency on six wouldn't be
detected by tox runs.

However, as other projects and distributions have phased out Python
2.7 support the dependency on six becomes less common. Thus, as long
as ceph-ansible does require it for config_template.py, add it to the
base requirements.

Signed-off-by: Florian Haas <florian@citynetwork.eu>
(cherry picked from commit d49ea9818b162a4b8fc0f5099f45acff4d22fe14)