Nova api service doesn't handle SIGHUP properly

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

Closes-Bug: #1334647

Change-Id: I3f7bd0ba5ef1bb716a97635252abe251053a669d
(cherry picked from commit 224a3de466d48301ab638b60ca47d19ad6b1b1dc)
Merge "Nova api service doesn't handle SIGHUP properly" into stable/icehouse