Add support for ELB Connection Draining attribute.

Manually tested version of ELB Connection Draining support for boto.  Here is the test script I used:

```bash
[ec2-user@dev-dkimdon boto]$ cat drain-test.py 
#!/usr/bin/env python

import boto
import sys

name = sys.argv[1]

draining_enabled = sys.argv[2] == 'true'
draining_timeout = sys.argv[3]

elb_conn = boto.connect_elb()
elbs = elb_conn.get_all_load_balancers()
for elb in elbs:
   if elb.name == name:
      print 'BEFORE'
      print elb_conn.get_all_lb_attributes(elb.name)
      conn_draining = elb_conn.get_lb_attribute(elb.name, 'ConnectionDraining')
      print conn_draining

      conn_draining.enabled = draining_enabled
      conn_draining.timeout = draining_timeout

      elb_conn.modify_lb_attribute(elb.name, 'ConnectionDraining', conn_draining)
      print 'AFTER'
      print elb_conn.get_all_lb_attributes(elb.name)
      print elb_conn.get_lb_attribute(elb.name, 'ConnectionDraining')

[ec2-user@dev-dkimdon boto]$ ./drain-test.py awseb-e-3-AWSEBLoa-1OLPI5HBDGUH0 true 200
BEFORE
LbAttributes(CrossZoneLoadBalancingAttribute(False), AccessLog(False, None, None, None), ConnectionDraining(True, 242))
ConnectionDraining(True, 242)
AFTER
LbAttributes(CrossZoneLoadBalancingAttribute(False), AccessLog(False, None, None, None), ConnectionDraining(True, 200))
ConnectionDraining(True, 200)
[ec2-user@dev-dkimdon boto]$ ./drain-test.py awseb-e-3-AWSEBLoa-1OLPI5HBDGUH0 false 200
BEFORE
LbAttributes(CrossZoneLoadBalancingAttribute(False), AccessLog(False, None, None, None), ConnectionDraining(True, 200))
ConnectionDraining(True, 200)
AFTER
LbAttributes(CrossZoneLoadBalancingAttribute(False), AccessLog(False, None, None, None), ConnectionDraining(False, 200))
ConnectionDraining(False, 200)
[ec2-user@dev-dkimdon boto]$ 
```

Fixes #2173.