Feature request: specify AWS credentials either with aws_profile item in aws section of config, or with --profile command line option

**Environment:**
-aws-parallelcluster==2.4.0
- RHEL 7
- slurm
- None
- None

I would like to be able to specify the AWS credentials through the use of a profile `~.aws/credentials`, as is done with most other AWS utilities. In the case of pcluster, this could be achieved by adding `aws_profile` item in the `[aws]` section of the configuration file, or by adding support for the `--profile` command-line option.