fixes #6445 - Configure external authentication via FreeIPA.

With these changes, ```foreman-installer --foreman-enable-ipa=true``` will configure external authentication using FreeIPA per http://theforeman.org/manuals/1.5/index.html#5.7ExternalAuthentication, provided the machine where Foreman is being installer (or reconfigured) is IPA-enrolled and HTTP/foreman-fqdn service exists in the FreeIPA server.
I'd prefer not to introduce another dependency of which this code would be the sole user. On the other hand, puppet/util/inifile seems already there, so how about the following?

```
require 'puppet/util/inifile'
Facter.add(:default_ipa_realm) do
  setcode do
    ini_file = Puppet::Util::IniConfig::File.new()
    ini_file.read('/etc/ipa/default.conf')
    global_section = ini_file['global']
    if global_section
      realm = global_section['realm']
      if realm
        realm.gsub!(/\s/, '')
      end
    end
  end
end
```