check if generated prometheus config is valid
via promtool.

This should help prevent bugs like:
https://bugs.launchpad.net/kolla-ansible/+bug/2076660

prior art: haproxy config validation:
https://review.opendev.org/c/openstack/kolla-ansible/+/922840



also add some basic documentation for the
`kolla-ansible validate-config` command.

Signed-off-by: Sven Kieske <kieske@osism.tech>
Change-Id: Ief90861b2c422e0e6c2dd9cb605c94e86c0f2ba1