Fix bug for switching to s3v4
Add S3v4 presigner

Allows you to create presigned urls for S3.
Add a general sigv4 presigner

Refactor the s3 v4 signer to use the general sigv4 presigner
where possible.
Consolidate signing tests into a single test module

This corresponds to a single botocore module and puts
all the signing tests in a single test module, with the
exception of SigV4, which is separate because it has its
own data driven test suite, given from the AWS docs.
Add sigv4 presigned unit/integ tests
Update EC2 model to the latest version
Remove ImportKeyPair customization

This is now in the upstream model so it's not needed anymore.
Add customization for EBS copysnapshot presigned url
Merge branch 'presign' into develop

* presign:
  Add sigv4 presigned unit/integ tests
  Consolidate signing tests into a single test module
  Add a general sigv4 presigner
  Add S3v4 presigner
  Fix bug for switching to s3v4
Merge branch 'ec2-update-ebs-enc' into develop

* ec2-update-ebs-enc:
  Add customization for EBS copysnapshot presigned url
  Remove ImportKeyPair customization
  Update EC2 model to the latest version
Fix integ test for py3