S3 Compatible:support RadosGW when using goamz in docker registry

[simpleshuai]

This pull request aims to S3 Compatible, associate with pull request distribution/distribution#902
This pull request fix two problem:

• send Relative URI instead of Absolute URI when working with RadosGW (mentioned by S3 Use tcpdump httprequest find something wrong #386 and Generates invalid URIs aws/aws-sdk-go#344 )
• complete upload multiple part add http header content-length when working with RadosGW(needed by RadosGW)

ps : when using "generic" as region name indicate that working with RadosGW

[alimoeeny]

Thanks @wangmingshuai , is there a way to add a test for this, so that future changes won't break it?

[simpleshuai]

hi @alimoeeny ,I added some tests for supporting radosgw ,and the pull request is #395 , can you review it ?