Cinder api service doesn't handle SIGHUP properly

When SIGHUP signal is sent to cinder-api service, it doesn't complete
processing of all pending requests before terminating all the
processes.

This change is a copy of Abhishek Kekane's fix for nova api.

Change-Id: I049d2aa2f3ad1fe388e00213a71f374803ed409a
Closes-Bug: #1334647
Merge "Cinder api service doesn't handle SIGHUP properly"