Use contextlib.nested for multiple 'with' clauses
Use contextlib.nested for multiple 'with' clauses

Multiple form of 'with' clause doesn't allow line break after eacn item.
It prevents pretty formatting of a big statement.

Nova widely uses contextlib.nested function for this purpose.

Change-Id: I928c5033064f08ec255a2818e11d9619afe749bc

hayderimran7/ec2-api,stackforge/ec2-api,openstack/ec2-api,stackforge/ec2-api,openstack/ec2-api,hayderimran7/ec2-api