setup.cfg: fix teuthology-openstack dependency

setup.cfg: fix teuthology-openstack dependency

The PR #2050 "Revive teuthology-openstack" introduced new dependency
after refactoring partly of teuthology.openstack instance classes
to use api instead of command line. However the openstacksdk
added to wrong section options.extras_require.openstack, that
intended to be used for provisioning openstack nodes via libcloud,
which is optional and is not installable by default.

I.e. `pip install -e ".[openstack]"` must be used in order to get it.

However the refactoring forced unconditional openstack requirement.

That's why this dependency should go to options.install_requires in order
to make `pip install -e .` work again.

Fixes: 252ab4f6a44c0d64b1e4cfb2b86e9bf58b5d9ffb
Signed-off-by: Kyr Shatskyy <kyrylo.shatskyy@clyso.com>