Set the argument type for ec2_vol's encrypted parameter

If this is not set, Ansible parses the parameter as a string.
This is fine if the parameter is not provided by the caller, but
if it is set to False or True explicitly, ec2_vol receives this as
the string 'False' or the string 'True', both of which are truthy.

Thus, without this fix, setting the parameter results in encryption
always enabled.

lib/ansible/modules/cloud/amazon/ec2_vol.py

@@ -377,7 +377,7 @@ def main():
             volume_size = dict(),
             volume_type = dict(choices=['standard', 'gp2', 'io1'], default='standard'),
             iops = dict(),
-            encrypted = dict(),
+            encrypted = dict(type='bool', default=False),
             device_name = dict(),
             zone = dict(aliases=['availability_zone', 'aws_zone', 'ec2_zone']),
             snapshot = dict(),