test(functional): Don't use a dead test server

It appears that when running in the gate, the wsgiref server sometimes
goes away. This adds a check to only reuse a server instance if it
is still alive.

Change-Id: I87716f9feb80da63a6b8c1f629178b396dbeb4c6

marconi/tests/functional/base.py

@@ -52,8 +52,8 @@ class FunctionalTestBase(testing.TestBase):
         self.mconf = self.load_conf(self.cfg.marconi.config)
 
         # NOTE(flaper87): Use running instances.
-        if (self.cfg.marconi.run_server and not
-                self.server):
+        if self.cfg.marconi.run_server:
+            if not (self.server and self.server.is_alive()):
                 self.server = self.server_class()
                 self.server.start(self.mconf)
 
@@ -179,7 +179,6 @@ class Server(object):
 
     __metaclass__ = abc.ABCMeta
 
-    servers = {}
     name = "marconi-functional-test-server"
 
     def __init__(self):
@@ -198,6 +197,14 @@ class Server(object):
         :returns: A callable object
         """
 
+    def is_alive(self):
+        """Returns True IFF the server is running."""
+
+        if self.process is None:
+            return False
+
+        return self.process.is_alive()
+
     def start(self, conf):
         """Starts the server process.