Add oom adj score attribute

Objective is to provide a method of influencing the targeting of redis during a low memory triggering of the out of memory killer process (oom_killer).  Depending on the version of the kernel, an appropriate value can be provided to adjust the calculated score.

Rationale for adding this is that we do not want a user caused out of memory event to kill redis in preference to other user processes.  By default, the value is nil and so does not change behaviour for existing cookbook users.