Validate and limit names for PutRule

[ssm951]

I noticed for resources with really long name, appending 8 random characters would make the PutRule name too long, causing an exception like the following:

An error occurred (ValidationException) when calling the PutRule operation: 1 validation error detected: Value '<SomeLongNameOver64Characters>' at 'name' failed to satisfy constraint: Member must have length less than or equal to 64

It may be better to truncate the Name string below if it has more than 56 characters, and then append the 8 random characters to ensure the name is less than 64 characters:

custom-resource-helper/crhelper/resource_helper.py

Line 270 in 3c7e40e

Name=self._event['LogicalResourceId'] + self._rand_string(8),