fix mms-agent recipe to not clobber the whole installation dir:

The existing installation process blows away its installation dir, which is not
polite if it's /usr/local/share (which is the default); plenty of other
software installs stuff there.

Changes:
- mms_agent.install_dir now correctly points to the actual install dir, and not
  its parent
- use chef's file cache instead of /tmp
- improved handling of installation dir
- both the installation and use of munin is now configurable via attribute