Mark this as a string so it is rendered in the docs

When this was treated as a boolean, sphinx was leaving the Default
column on http://docs.ansible.com/ansible/ec2_module.html blank,
implying it would use AWS's default.  In reality, it passes False, which
overrides the defaults at AWS (it's possible to boot an instance which
AWS claims will always have EBS optimization without it because of this
silently passed False).

cloud/amazon/ec2.py

@@ -225,7 +225,7 @@ options:
     description:
       - whether instance is using optimized EBS volumes, see U(http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSOptimized.html)
     required: false
-    default: false
+    default: 'false'
   exact_count:
     version_added: "1.5"
     description: