Allow getting users and keys from another AWS account

[mvanbaak]

Implements #21

When the AssumeRole variable in both scripts is set to the ARN of a role in another account, use that role to get the users and keys.
The usecase is described in the issue:

• An AWS account with all the users in IAM (eg management account)
• One or more AWS accounts that run the EC2 instances (following AWS best practices)

The InstanceRole attached to the EC2 instance(s) should have permissions to assume this role (example given in the showcase CF template) and the 'management' account should have setup this role and allow the EC2 accounts access to assume this role.

The two shellscripts are now running on our instances (in test still, but it works).
The showcase CF template has not been tested, but I think this should be it. But it can be broken, it needs testing (time is short, maybe later I can test it)

Documentation is lacking, but I hope it's clear in the description.

[michaelwittig]

#24 was merged as discussed in #28