Support aws.profile JVM system property

[aajtodd]

The default AWS profile name is [default] which can be overridden by the environment variable AWS_PROFILE as well as the JVM system property aws.profile.

Currently the CRT respects the environment variable when parsing profiles including when constructing credentials via the default chain.

Task is to add support for our ProfileCredentialsProvider to respect this JVM system property when specified.

[aajtodd]

closed by #312