Prevent wrongly privilege escalation of a context

Current codes in create_volume() may exists a scenario that
a context which is not elevated but after it fails in volume
creation, it becomes elevated. This patch saves original
context, so that if it fails in volume creation simply use the
original context for further scheduling.

Fix bug:1187076

Change-Id: I2822b1612ec741209b278fc65f18d0f8f3243e0a