Inline a custom file_line provider to fix agent.

Fixes an issue in the ceilometer::agent::compute manifest
where nova.conf config values for the notification_driver
would get added to the wrong section (always to the end of
the file).

As part of the fix a custom file_line 'after' provider which
supports a new after option has been added.
This allows us to have some control over which section *new*
file lines go into. If there are any pre-existing matching
lines in the file the assumption is that they are already in
the correct section and can be edited in place.

NOTE: I've submitted a pull request to the upstream stdlib repo here
to add the new 'after' option:

  https://github.com/puppetlabs/puppetlabs-stdlib/pull/174

Once this (or something better) lands in stdlib we can update
puppet-ceilometer to use it.

Fixes LP Bug #1217867

Change-Id: Ic09f5232b322cde687d663d1ef38ef0fd12f32ff