include a sanitized version of unit_name and relation name in Ansible's host_vars

Right now, I'm using something like this:

- name: set sanitized_unit_name
  set_fact: sanitized_unit_name="{{ local_unit | regex_replace('^(.*)/.*$', '\\1') | regex_replace('-', '_') }}"

- name: get relation name
  set_fact: relation_name="{{ ansible_env.JUJU_RELATION | regex_replace('(.*)-relation-.*','\\1') }}"
  when: ansible_env.JUJU_RELATION is defined

Since we do that a lot in charm it would be useful to do it in the library directly.