Default to nil p1_file for source installs

This patch contains three fixes:

1) define default directory attributes above the "platform specific attributes" since some of those platform specific attributes depend on the directory attributes being defined

2) it sets a p1_file attribute for the debian platform family, and in the server_source recipe, the `p1_file` default is set to nil

3) the p1_file attribute in the `nagios.cfg` file is only included if the user has a `p1_file` set.