add PyYAML to package

[oniGino]

onceover complains that this python module is missing for linting hiera config
please add
RUN yum install PyYAML -y

see:

+ onceover run codequality --no_lint
INFO	 -> Running Code Quality tests
INFO	 -> Checking Puppetfile...
INFO	 -> ...OK
INFO	 -> Checking puppet-syntax rake task...
INFO	 -> ...OK
WARN	 -> Please install python and pyyaml for enhanced YAML validation (pip install pyyaml)
INFO	 -> Checking Generate documentation in site/role/...