Basic instance interop helpers added.

Partially implements: blueprint instance-interop-helper.

Change-Id: I453ddce3e5253880a000fcb287163d13803797a3
Use six to improve python 3 compatibility.

* StringIO

Change-Id: I8471e525566a0353d9276529be4b0d0e0cbf6cd6
Hostname/username are now available in Instance.

Change-Id: I2d27ca263d32212290b0c9c885866ffa52d5cc78