Allow complex scheduler hints for AWS Instance

Nova supports providing several values for the same scheduler hint,
in which case the hint value is formed as a list. This functionality was
missing in AWS::EC2::Instance resource.

Change-Id: Id05e4f2ebe89871c1f27f8d97441f3b6cf469dd3
Closes-Bug: #1297667
Merge "Allow complex scheduler hints for AWS Instance"