Add functions & cleanup install_puppet.sh

A small clean-up of the install_puppet.sh script to enhance
readability and to simplify adding support for extra distributions

 - add distribution check functions
 - move puppet installation for each distribution into its own
   separate function
 - move pip installation into a function
 - add/expand several comments

Change-Id: I5d5f71fde607ace528b7372e2deadccbea58bd2f
Initial RHEL7/Centos7 support

Install the right EPEL and puppet rpm's for RHEL7/Centos7.  I have
tested this on one of the CentOS nightly builds and it seems to
install puppet correctly, and install_modules.sh works.

Change-Id: I80024d1afdb4e40d5fe9793ab2ec443b887c5fa8