Add support for installing agent via official packages

Added support for installing the agent via official packages from repo.zabbix.com.
Attributes:
* `node['zabbix']['agent']['install_method'] = 'package'` -- install via package
* `node['zabbix']['major_version']` -- Major version to install. Defaults to 2.2

Updated Vagrantfile to install agent via package, added a box `zabbix-src-agent` to be able to test source install as well.