Add a generic "fill_field" method to the page model

- Based on a method of the same name from aweiteka's ldap_config branch
  https://github.com/aweiteka/cfme_pages/blob/ldap_config/pages/configuration_subpages/settings_subpages/server_settings_subpages/authentication_settings_tab.py#L75
- Returns the filled element instead of selenium's send_keys return, as send_keys will always return None