Add the collectd df plugin

This defaults to collecting inode counts, which is needed for adding an
alert around running out of inodes.
Merge pull request #548 from alphagov/add-collectd-df

Add the collectd df plugin
Remove df from collectd plugin list

Commit cccf932 made use of the df defined type so it has to be removed
from the raw collectd::plugin list.
Merge pull request #549 from alphagov/fix-collectd-df

Remove df from collectd plugin list