Add support for yum commands

[VirtualTim]

It looks like there are a number of really useful tests for apt-get commands, but currently no support the the equivalent yum commands.
This would obviously be really useful for anyone using a CentOS/Redhat based image.

At a minimum I'd suggest:

• yum install must have a -y
• yum install shoudl always have a yum clean all, rm -rf /var/cache/yum and rm -f /anaconda-post.log in teh same RUN.