add 'stop instance' operation before volume detach
add 'stop instance' operation before volume detach

to prevent volume stuck in busy state.
From AWS: Make sure to unmount any file systems on the device
within your operating system before detaching the volume. Failure
to do so results in the volume being stuck in a busy state while detaching.

Change-Id: I6e96c0125a07c09b5f36a9e086d58075cc47e98b

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