Consistent _get_multi_param() function in responses

This abstracts _get_multi_param() into BaseResponse and makes it
always ensure that the string it has been given ends with a '.'.  It
had been implemented in three different places, and in use it rarely
postpended a trailing period, which could make it match parameters it
shouldn't have.

moto/ec2/responses/instances.py

@@ -8,10 +8,6 @@ from moto.ec2.exceptions import InvalidIdError
 
 
 class InstanceResponse(BaseResponse):
-    def _get_multi_param(self, param_prefix):
-        return [value[0] for key, value in self.querystring.items()
-                if key.startswith(param_prefix + ".")]
-
     def describe_instances(self):
         instance_ids = instance_ids_from_querystring(self.querystring)
         if instance_ids: