Fix default postrun_command

The r10k::postrun_command command parameter was pointing to a
non-existent variable in params.  This commit resolves this by setting a
"pre_postrun_command" in params.pp that both the prerun_command and
postrun_command classes use as default values for their 'command'
parameters.
Merge pull request #63 from joshbeard/postrun_fixes

Fix default postrun_command