Allow the specification of the mode for $commitlog_directory, $data_file_directories and $saved_caches_directory

[dallinb]

This is really an extension to issues #104 and #109. By default the modes should be 0750 but on Debian as part of the service startup, this was changed to 0770. Therefore on Debian, the options are:

a) Run the manifest at least twice before an idempotent run will work.

or

b) Specifically set the modes to 0770.

[dallinb]

While at it, fix the following untested resources:

Total resources:   320
Touched resources: 316
Resource coverage: 98.75%
Untouched resources:
  File[commitlog_directory]
  File[datadir1]
  File[datadir2]
  File[saved_caches_directory]

See https://travis-ci.org/locp/cassandra/jobs/81532858 for details.