Add new context manager for shade exceptions, final.
Add new context manager for shade exceptions, final.

This commit completes the work of
 change Ic39b6e3efc33b7a30f298a3daca6d6c16ab4ca89

Shade has developed a common pattern within many of the API
methods where we try to avoid re-wrapping OpenStackClientException
exceptions by catching them and simply re-raising. We can eliminate
this duplicate exception handling with a context manager.

Change-Id: If236656146ecfa5a2b3eb3379efc6869829bdaa7

openstack/python-openstacksdk,dtroyer/python-openstacksdk,stackforge/python-openstacksdk,stackforge/python-openstacksdk,openstack-infra/shade,openstack/python-openstacksdk,openstack-infra/shade,dtroyer/python-openstacksdk