Agent config changes

This commit modifies the agent configuration in two ways:

First, it removes pre and postrun ubuntu commands. These are
installed by default with the standard Ubuntu repository, but
not with the Puppetlabs repository. Since these options are
configured to be present and there's no way to remove them
through this module, it might be best if they are removed.

Second, $report_server and $ca_server are renamed to $real_* to
satisfy the default fallback value of $server.
Further agent configuration changes

report_format will not be set if it is not specified.

All agent-related settings should now be configurable through
the agent manifest rather than enforcing various default values.
Reverting some agent changes

Some configuration for puppet.conf's main section is already
done in manifests/init.pp. This commit restores that functionality.
Merge pull request #47 from jtopjian/jtopjian-agent-changes

Agent config changes