Allow for reuse of allocated but unassociated Elastic IPs.

I've added a parameter ```reuse``` to the ec2_eip module describing if unassociated Elastic IPs can be reused. It's not always necessary to allocate a new one, hence this parameter. 

I'm aware that reuse can be dangerous too, since an unassociated Elastic IP might be parked to be used later, so it should be used with caution. When set to false, which is the default as well, the module behaves as it did.