Session object can be created with session id
Session object can be created with session id

Details:
- A session object be created by a existing session id.
- Remove double assignment of _header and session_id.

Signed-off-by: Juergen Leopold <2ef4e6fc356c4a0c570adb4e83b64b658f380892@de.ibm.com>

zhmcclient/python-zhmcclient,zhmcclient/python-zhmcclient