rsyslog: make all module params configurable

+ support false for *_package_name variables. This is useful if relp
support is not necessary as it happens that no packages exist for
RedHat < 6.

All these changes should be fully backward compatible.
use rsyslog::snippet for client and server configs

Doesn't feature any functional change, but it seems cleaner to use
that definition to avoid repeating dependencies and paths.
Merge pull request #1 from ckaenzig/configurable-params-and-misc

Configurable params and misc