USE of AWS_PROFILE

[cell]

Two behaviours adapted:

• If AWS_PROFILE is set, it will be used as default value for the user/profile. Of course if the option -u is specified, its value is taken.
• If a user/profile is given, be it by environment variable or argument, awsudo will use its value to set a new aws variable: AWS_PROFILE.

[alecraso]

LGTM, can you add tests?

[cell]

@aaronbiller It tooks some time but here are the tests.