fix(contrib/ec2): restrict EBS type to standard or gp2

Provisioned iOPS require specifying more options, and accommodating
that logic can be left to another PR.

Author: carmstrong

contrib/ec2/deis.template.json

@@ -82,8 +82,8 @@
       "Description" : "EC2 EBS VolumeType",
       "Type": "String",
       "Default": "gp2",
-      "AllowedValues" : [ "gp2", "io1", "standard" ],
-      "ConstraintDescription" : "must be either 'gp2' for SSD, 'io1' for guaranteed IOPs, or 'standard' for magnetic"
+      "AllowedValues" : [ "gp2", "standard" ],
+      "ConstraintDescription" : "must be either 'gp2' for SSD or 'standard' for magnetic (provisioned iOPS requires further manual changes)"
     },
     "AssociatePublicIP": {
       "Description": "Whether to associate a public IP address to the instances",