Make mn -w wait indefinitely and add --twait option (#1036)

Background: In Mininet 2.2, waitConnected() waits forever
by default. We are going to preserve this behavior for 2.3.

Therefore, the --wait/-w option will wait forever. This is used
in the tests to make sure that all switches have connected to
their controllers.

A new --twait/-t <int> option has been added for timed waits.

The API for Mininet(....waitConnected=False/True) is preserved,
but you can now pass in an integer wait time. False means
do not wait at all. True means wait forever. I have elected
for now to preserve None also meaning wait forever as it was
in 2.2, but note that you should probably use the boolean
True/False instead.

bin/mn

@@ -290,6 +290,9 @@ class MininetRunner( object ):
                          help='prints the version and exits' )
         opts.add_option( '--wait', '-w', action='store_true',
                          default=False, help='wait for switches to connect' )
+        opts.add_option( '--twait', '-t', action='store', type='int',
+                         dest='wait',
+                         help='timed wait (s) for switches to connect' )
         opts.add_option( '--cluster', type='string', default=None,
                          metavar='server1,server2...',
                          help=( 'run on multiple servers (experimental!)' ) )