Validate the display parameter in display::env

This commit validates that the value for the 'display' parameter in
display::env is an integer.
display::env should inherit params

Since we're setting the 'display' parameter to a default obtained from
display::params, we need to inherit this class.
Validate runuser parameter for x11vnc

This commit validates that the value for the 'runuser' parameter is a
string.
Fail with osfamily message, not operatingsystem

Since our case statement is evaluating the $::osfamily fact, our fail
message should report THAT instead of $::operatingsystem.
Only validate display_env path if its set
Merge branch 'master' of github.com:joshbeard/puppet-display