Implement -k option for cfn-get-metadata

The -k argument can be a top-level key or a nested key in which case
the keys are separated by dots (eg "foo.bar"). In case a key contains a
dot character, it needs to be surrounded by single quotes (eg
"foo.'bar.1'.fred).
If the -k option is not provided, the command prints out the full
metadata structure as before.

Change-Id: Ib05d39672086001b83e8d7f56bc42cc4ba75751c
Fixes: bug #1183299