Support configuration per command group

[fkrull]

Currently, configuration settings like command properties can only be configured either globally or per command. This makes it difficult to configure, say, command timeouts per service without manually adding configuration settings for every command or programmatically loading common settings during HystrixCommand construction.

Proposal: an additional level of properties should be supported, hystrix.command.HystrixCommandGroupKey...., which overrides the global settings, but gets overridden by command-specific settings.