Nova-api service throws error when SIGHUP is sent

Added reset method in WSGIService class.

After adding reset method when SIGHUP signal is sent to
wsgi service parent process,then it sends SIGHUP signal
to all of its child processes. Each child process handles
SIGHUP signal by first stopping the service and then calls
service start method again. When it stops the service, it
kills the eventlet thread, which internally closes the wsgi
server socket object. This server socket object is now not
usable again and it throws following error, while restarting
the service:

error: [Errno 9] Bad file descriptor

To resolve 'Bad file descriptor' error, creating duplicate
socket object, every time service starts.

When the wsgi service is stopped, it sets the green pool
size to 0, so resizing the green pool to default value,
on service restart.

Closes-Bug: #1334651

Change-Id: If1035deaf8b31f2712d88f0112fdd2e9e9dc7cb0
Merge "Nova-api service throws error when SIGHUP is sent"