app: break provisionApp in two smaller actions

This make the pipeline more atomic (and solves one of my TODOs).
app: split bucket and IAM credentials credential

Starting the refactoring that will remove that last TODO in the
actions.go file.
app: rename files

Calling these files s3.go and s3_test.go does not make much sense. They
are not related to S3 only, but also to IAM. I could use "s3iam", but I
preferred just calling it aws{_test}.go.
app: update license header in aws related files

Also improved a test.
app: break createBucketIam in smaller actions

It became 5 (!) actions (I predicted 4):

	1. create user in IAM
	2. create access key for the user in IAM
	3. create the bucket in S3
	4. create the user policy for accessing the bucket in IAM
	5. export tsuru's default environment variables in the app (the
	name of the app, host of tsuru server and S3 related stuff -
	access key ID, secret access key, bucket name, endpoint and
	location constraint rule).

This is a huge commit, but it's entirely reversible and actually a
"small" change.
app: removed "useQueue" parameter from UnsetEnvsFromApp

This method never used the queue, this parameter was never used.
app: use a goroutine for serializing environment variables

Just avoid blocking while waiting for IO.
app: remove dead code

This code looks stupid (and it is), but it's a legacy from the time that
we had two EnvVar types: one declared in the bind package, and other in
the app. So, SetEnvs existed to satisfy the interface bind.App, and used
SetEnvsToApp that received app.EnvVar, that's why the conversion
existed.

When we changed the implementation to use only bind.EnvVar, we probably
run some kind of sed or gofmt -r, and the lack of review allowed such
stupid code to exist.

Well, not anymore :-)
app: clean API

Unifying UnsetEnvs and UnsetEnvsFromApp. Unfortunately, it's not
possible to do the same with SetEnvs and SetEnvsToApp, because
SetEnvsToApp have that third parameter.
app: use a copy of app in exportEnvVarsAction test to avoid data race