Changes file_path to self.file_path

Changes file_path to self.file_path in ceilometer_config provider
in order to allow purging as specified in ini_file puppet module
readme.

Change-Id: I41f7c83e3a63b9af9e683933aa09bd45d5b18deb
Closes-Bug: 1280329
Merge "Changes file_path to self.file_path"