fix the issue that host style is not effect

[xiaozhu36]

This PR aims to fix two issues:

1. HostStyle is not effect
2. Can not support China region and Non-China region at the same time when provider is alibaba cloud.

This PR does the two things:

1. Re-set v2.HTTPRequest.Host when HostStyle is true. The prefect solution is aws-sdk-go PR
2. Setting header date according to config region

[cfdreddbot]

✅ Hey xiaozhu36! The commit authors and yourself have already signed the CLA.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/161882697

The labels on this github issue will be updated when the story is started.

[bosh-admin-bot]

This pull request was marked as Stale because it has been open for 21 days without any activity. If no activity takes place in the coming 7 days it will automatically be close. To prevent this from happening remove the Stale label or comment below.

[bosh-admin-bot]

This pull request was closed because it has been labeled Stale for 7 days without subsequent activity. Feel free to re-open this pull request at any time by commenting below.