Fixes TRAINVM-46: check hostname validity

See: https://tickets.puppetlabs.com/browse/PUP-677
Puppet fails when the hostname, and thus the username on the master
  is set to all numerals.
I decided to apply the test to advanced as well, since although we
  don't manage the ssh_authorized_keys it is possible that something
  that relies on executing a command as a user might fail if we set
  the hostname and the username with all numeric characters.
copy+paste fix
Disabling the caching of PE pacakges

yum-enterprise.puppetlabs.com is no longer needed for PE > 3
The puppetlabs yum repo, yum.puppetlabs.com now disabled by default
Merge pull request #184 from casharma/fix/remove-yum-enterprise

Disabling the caching of PE pacakges
update the regular expression for hostname validity
Fixes TRAINVM-56
Merge pull request #171 from casharma/bugfix/TRAINVM-46

Bugfix/trainvm 46
Additional steps needed to disable puppetlabs-enterprise-extras
Merge pull request #185 from casharma/more/disabling_enterprise_extras

Additional steps needed to disable puppetlabs-enterprise-extras