ipaserver: Add support for external signed CA

This adds support for the --external-ca option to ipaserver. Lots of
additional tests and checks from ServerInstallInterface.__init__ have
been added to ipaserver_test. Also duplicate tests cna checks have been
removed.

Installer settings in ansible_ipa_server module_util are now also set
to the defaults that are used in Installable, ServerInstallInterface,
ServerMasterInstall, ADTrustInstallInterface and Uninstall.

The /root/ipa.csr file generated on the node in ca.install_step_0 will
be copied to the controller as "{{ inventory_hostname }}-ipa.csr".

The new task file copy_external_cert.yml has been added to copy the
generated certificate defined in ipaserver_external_cert_files to the node
to continue with ca.install_step_1.

The tasks/install.yml file has been adapted to make sure that the steps
that will be done in step two will be skipped after step one has been
done.