Update the puppet snippet to support --server and 'puppet agent'.

Issue #475
This patch also adds two new settings to /etc/cobbler/settings:
* puppet_server: specify a --server param in puppet snippet.
* puppet_version: specify 3, to have snippet use 'puppet agent'.

All of these changes default to the historical status quo.
Merge pull request #484 from jimi1283/issue_475

Update the puppet snippet to support --server and 'puppet agent'.