move the directory config up, unset p1_file for source installs
put the attribute default override in the recipe

This seems like a better place for the source-install specific default
than the attributes file, since we want the default p1_file attribute
set as close as possible to the install process... I think.
use default instead of set_unless
Merge pull request #270 from caleb/p1-file

Default to nil p1_file for source installs