Add support for AWS_CONTAINER_CREDENTIALS_FULL_URI env variable

[t089]

Is your feature request related to a problem? Please describe.
When using aws-vault to provide temporary AWS credentials, it exposes a metadata service and puts its address in the AWS_CONTAINER_CREDENTIALS_FULL_URI env variable. Currently, the metadata credentials provider does not know about this variable.

Describe the solution you'd like
If set, the metadata credentials provide should use the url stored in AWS_CONTAINER_CREDENTIALS_FULL_URI to fetch credentials.

[adam-fowler]

This doesn't look too hard to add.