Allow graceful shutdown on Ctrl+C
Allow graceful shutdown on Ctrl+C

Currently it prints a stacktrace and exits. This patch makes it print
a message and exit gracefully.

Closes-Bug: #1256732

Change-Id: I59ae8d09be43cd3e56dbe9bed4f6f328377dcc13

mat128/python-novaclient,guptaankita/python-novaclient,CCI-MOC/python-novaclient,akash1808/python-novaclient,Shihta/python-novaclient,metacloud/python-novaclient,guptaankita/python-novaclient,metacloud/python-novaclient,JioCloud/python-novaclient,takeshineshiro/python-novaclient,CCI-MOC/python-novaclient,JioCloud/python-novaclient,akash1808/python-novaclient,takeshineshiro/python-novaclient,Shihta/python-novaclient,mat128/python-novaclient