Allow configuring the wsgi pool size

Expose the number of greenthreads in wsgi pool to configuration. This
allows to put a limit to the number of resources like memcache
connections used.

DocImpact
Change-Id: I6e53d6e12661ed2f42a6b543891b13eec0c8300c

etc/nova/nova.conf.sample

@@ -306,6 +306,10 @@
 # socket. Not supported on OS X. (integer value)
 #tcp_keepidle=600
 
+# Size of the pool of greenthreads used by wsgi (integer
+# value)
+#wsgi_default_pool_size=1000
+
 
 #
 # Options defined in nova.api.auth

nova/wsgi.py

@@ -64,7 +64,10 @@ wsgi_opts = [
     cfg.IntOpt('tcp_keepidle',
                default=600,
                help="Sets the value of TCP_KEEPIDLE in seconds for each "
-                    "server socket. Not supported on OS X.")
+                    "server socket. Not supported on OS X."),
+    cfg.IntOpt('wsgi_default_pool_size',
+               default=1000,
+               help="Size of the pool of greenthreads used by wsgi"),
     ]
 CONF = cfg.CONF
 CONF.register_opts(wsgi_opts)
@@ -75,7 +78,7 @@ LOG = logging.getLogger(__name__)
 class Server(object):
     """Server class to manage a WSGI server, serving a WSGI application."""
 
-    default_pool_size = 1000
+    default_pool_size = CONF.wsgi_default_pool_size
 
     def __init__(self, name, app, host='0.0.0.0', port=0, pool_size=None,
                        protocol=eventlet.wsgi.HttpProtocol, backlog=128,