tests: Create IPA fact ipa_host_is_client.

This patch adds a new fact `ipa_host_is_client`, available in the
include task file `env_freeipa_facts.yml`.

The variable `ipa_host_is_client` is set to TRUE if the client host
is not an IPA server (primary or replica). It can be used if a test
cannot be executed in a server, independent of the value set to
`ipaapi_context`.