Do not set special_service_provider on RHEL7.

RHEL7 uses systemd and should set this to undef as Fedora does.

Change-Id: Ic9a719dfff50bda85d428fba5d665d7b97d8959c
Merge "Do not set special_service_provider on RHEL7."
Merge git://github.com/puppetlabs/puppetlabs-nova