test cfn_helper.to_boolean
test cfn_helper.parse_creds_file
Pass paths into Metadata.retrieve for testing.
Tests for Metadata.retrieve()

This doesn't test remote retrieve yet.
Don't conceal any exception raised by describe_stack_resource.

The raised MetadataServerConnectionError had weird
whitespace in the message too.
Test retrieval of remote metadata and call to cfn_init.
Testing of each handler will come separately