Don't look at region vars for global endpoints

If a profile is specified, then trying to access a global endpoint
will result in an exception about the endpoint either not being valid,
or if a profile doesn't have a region, then it will complain that
there's no region set for the instance.

An example of how a user can encounter this:

* Have a default region of us-west-2 via `export AWS_DEFAULT_REGION=us-west-2`
* Have a config file with a profile that just has access/secret keys (no region var):

```
[profile production]
aws_access_key_id =foo
aws_secret_access_key=bar
```

Trying to access a global endpoint such as IAM works without a profile specified:

```
$ aws iam list-groups
[ ... response ...]
```

But if you specify a profile you can an error:

```
$ aws --profile production iam list-groups
Service iam not available in region us-west-2
```

This PR fixes this last case.