Inconsistent uuid generation in azure_arm

## Summary

Azure returns inconsistent casing of the resource groups name in the id string, resulting in different uuid's for the same node (depending on what calls you make).

## Detailed Information

When a node is created, the resource groups name in the id string is lower case. In subsequent calls, like in list_nodes(), for some reason Azure returns the resource groups name in upper case. This results in a different uuid from when create_node() is called. This also breaks wait_until_running(). Forcing the id string to lower case in _to_node() seems to resolve this issue.

I made the change here: https://github.com/foospidy/libcloud/commit/2682d1229466fa577d4360ababd650d777ea49f3

I'd be happy to submit a PR.

首先定位 Azure ARM provider 的 _to_node() 方法，并比较 create_node() 和 list_nodes() 如何生成 ID。验证 resource-group 的大小写不再改变节点的 UUID，并确认 wait_until_running() 的行为一致；以引用的 commit 作为上下文，如果 provider 的测试结构确定了合适的位置，则添加或更新针对性测试。

由索引模型根据 Issue 内容生成。