Updating elasticache parameters to be optional

With the addition of Elasticache for Redis, AWS has made certain
parameters to the endpoint optional. This commit reflects that change.
Adding a test for the required elasticache launch parameters
Fixing the dynamo 'scan in' filter
Merge pull request #1876 from stevearc/elasticache_api_fix

Updating Amazon ElastiCache parameters to be optional when creating a new cache cluster. Fixes #1876.
Merge pull request #1896 from stevearc/dynamodb_scan_in

Fix the DynamoDB 'scan in' filter to compare the same attribute types in a list rather than using an attribute set. Fixes #1896.
Make trailing slashes for metadata URLs explicit and remove the trailing slash from userdata calls. This is an alternative implementation to #1897.
Fix example for building metadata URL
Merge pull request #1900 from danielgtaylor/metadata

Make trailing slashes for EC2 metadata URLs explicit & remove them from userdata requests. Fixes #1900, #1897, #1856.