move python-xml to raw_install_python.yml

move python-xml to raw_install_python.yml

The package python-xml is needed for ansible's zypper module to interact with
the zypper package management tool.

roles/ceph-defaults/defaults/main.yml:
Remove python-xml from variable suse_package_dependencies to only
install python-xml on SUSE/openSUSE if python is not found.
raw_install_python.yml already contains all the logic needed to check
if there is a valid python installation, so this is better suited there.

openSUSE Leap 15.x / SLES 15.x do no longer have /usr/bin/python,
only /usr/bin/python3, which already contains the xml module, so
nothing needs to be installed in that case.

Signed-off-by: Johannes Kastl <kastl@b1-systems.de>
(cherry picked from commit 5cf22e9b312bb26b3144c329e6e597a0905b274a)