Eliminate the race when selecting a port for tests.
    
    Functional tests currently select ports on which the various services
    will listen by opening a socket in the ephemeral port range, checking
    its port, closing the socket, and then sometime later starting the
    service and instructing it to use that port.  There are a few bugs that
    reference fragile tests.  In many of these case it is possible that
    another processes claimed the selected port in between the time it was
    first opened and closed and the time that the service actually used it.
    Because this is a known possibility, and because the tests fail
    infrequently resolving these bug becomes difficult.
    
    This patch eliminates the window in which another processes could claim
    the port.  The socket is opened in the test code but not closed. Then
    the service process (api or registry) is forked off where it inherits
    the open file descriptor.  A environment variable is used to tell the
    new wsgi process that it is a test process and should thus get its
    socket from the FD instead of creating a new socket.
    
    In the case where glance-control is used with the --respawn option this
    solution is not possible.  The problem is that the FD is only good once.
    I can pass from the test code, to glance-control, and to glance-{api,
    registry} but only 1 time.  If the service dies, and glance-control
    restarts it the FD will no longer be valid.  For this case the forked FD
    code is disabled.  However, the socket is still closed much later making
    the race condition even less likely.
    
    This fixes bug: 1047593
    It may fix some other "fragile test" bugs as well.
    
    Change-Id: I27313d144bc7bd2132a604dcc22916c80338abab