Fix password_file handling
MODULES-618 - fix java_ks when using password_file

Anywhere we were using @resource[:password] in the provider use
password_file instead. Make sure we try using password_file.

Fixes puppetlabs/puppetlabs-java_ks#59
Merge pull request #103 from mhaskel/MODULES-618

MODULES-618 - fix java_ks when using password_file