connection: fix logging of non-IP sockets

Before this fix, the debug log would crash _connect_socket for UNIX 
domain sockets. getsockname() for UNIX domain sockets returns a single 
string instead of a tuple (as is the case for IPv4/IPv6). Therefore the 
code could crash as it tried to get the second element of a non-tuple 
(empty string):

  Traceback (most recent call last):
    File "/home/margdoc/Workspace/scylla/maintenance_mode_testing.py", line 5, in <module>
      s = c.connect()
        ^^^^^^^^^^^
    File "cassandra/cluster.py", line 1750, in cassandra.cluster.Cluster.connect
    File "cassandra/cluster.py", line 1776, in cassandra.cluster.Cluster.connect
    File "cassandra/cluster.py", line 1763, in cassandra.cluster.Cluster.connect
    File "cassandra/cluster.py", line 3581, in cassandra.cluster.ControlConnection.connect
    File "cassandra/cluster.py", line 3642, in cassandra.cluster.ControlConnection._reconnect_internal
  cassandra.cluster.NoHostAvailable: ('Unable to connect to any servers', {'test_socket': IndexError('string index out of range')})

Fix the issue by not unpacking those values and just printing them
as-is, relying on %s formatter to print all elements of a tuple (host,
port) for IP sockets and string for UNIX domain sockets. The printed log
is not formatted as nice as before, however this is a DEBUG print 
so few users will ever see it. The new approach should work with any 
format of getsockname().

Fixes #278

cassandra/connection.py

@@ -940,7 +940,7 @@ def _connect_socket(self):
                 self._initiate_connection(sockaddr)
                 self._socket.settimeout(None)
                 local_addr = self._socket.getsockname()
-                log.debug('Connection %s %s:%s -> %s:%s', id(self), local_addr[0], local_addr[1], sockaddr[0], sockaddr[1])
+                log.debug("Connection %s: '%s' -> '%s'", id(self), local_addr, sockaddr)
                 if self._check_hostname:
                     self._match_hostname()
                 sockerr = None