Allow profile to be set via system property as an alternative

[neowulf]

Although, it's nice to execute:

AWS_DEFAULT_PROFILE="sandbox" sbt ecr:createRepository

it'd be nice to have an alternative to execute:

sbt -Daws.profile=sandbox ecr:createRepository

This also has a nice property that the build can be configured via the initialize sbt task which prevents for users to specify the profile by default if not found.

initialize ~= { _ =>
  System.setProperty("aws.profile", sys.props.getOrElse("aws.profile", "sandbox"))
}

[sjednac]

This could be a nice extension indeed. The main issue I'm seeing are priorities for the config - we should probably keep the same order as in the authentication chain.

I'll do some research on the existing conventions and I'll get back to you. Thanks for the contribution!