Issue #11: Provide query_api_version() which provides the results of GET /api/version
Issue #11: Provide query_api_version() which provides the results of GET
/api/version

Details:
- Provide a client method called query_api_version()
  which provides the results of GET /api/version.
- Cache query_api_version() results in self._api_version
  variable of client object.
- version_info() returns cached api-major-version and api-minor-version
  fliedds of self._api_version.

zhmcclient/python-zhmcclient,zhmcclient/python-zhmcclient