Insert missing safety check

Without this check allocate_new_eip becomes required on every ENI allocation,
which is frustrating and breaks this for no reason.

```
[DEBUG   ] MasterEvent PUB socket URI: ipc:///var/run/salt/master/master_event_pub.ipc
[DEBUG   ] MasterEvent PULL socket URI: ipc:///var/run/salt/master/master_event_pull.ipc
[DEBUG   ] Sending event - data = {'profile': 'staging-util-ext', 'event': 'starting create', '_stamp': '2014-05-29T11:13:00.147826', 'name': 'staging-util01', 'provider': 'aws:ec2'}
[INFO    ] Creating Cloud VM staging-util01 in us-east-1
[ERROR   ] Failed to create VM staging-util01. Configuration value 'allocate_new_eip' needs to be set
Traceback (most recent call last):
  File "/Library/Python/2.7/site-packages/salt/cloud/__init__.py", line 1162, in create
    output = self.clouds[func](vm_)
  File "/Library/Python/2.7/site-packages/salt/cloud/clouds/ec2.py", line 1819, in create
    data, vm_ = request_instance(vm_, location)
  File "/Library/Python/2.7/site-packages/salt/cloud/clouds/ec2.py", line 1281, in request_instance
    if interface['allocate_new_eip']:
KeyError: 'allocate_new_eip'
Error: There was a query error: Host for new master staging-util01 was not found, aborting map
```