Introduce the iburst_enable parameter to use "iburst" option

Hi,

this pull request makes the option "iburst" configurable. We use Debian and CentOS server and until now the CentOS server doesn't use "iburst" while the Debian server have it turned on by default.

The old default  behavior is preserved if no iburst_enable parameter is set.

The RSpec tests run successful but unfortunately i was not able to run the rspec-system tests. 

Best wishes,
Matthias