(MODULES-636) Allow version to be user-defined

Without this patch, the rabbitmq class defines a package source for
RedHat and Suse systems in params.pp, based on the version in
params.pp. This means that the version is not overrideable. This patch
moves the construction of the RPM package source to the rabbitmq class
so that it takes into account the version that a user has defined, if
any. It also adds clarification about the behavior of the
package_source and version parameters for systems that don't use RPM.