Add Icinga checks for important sysctl params

Add 'check_sysctl', an Icinga plug-in that issues alerts whenever the actual
and expected values of sysctl parameters are not the same.

I made two additional changes that were prompted by rereading the README of the
Debian procps package:

  "In general, files in the 10-*.conf range come from the procps package and
  serve as system defaults.  Other packages install their files in the
  30-*.conf range, to override system defaults.  End-users can use 60-*.conf
  and above, or use /etc/sysctl.conf directly, which overrides anything in
  this directory."

Thus:

* Make the default priority of sysctl::conffile and sysctl::parameters
  resources 60, rather than 10. Any additional sysctl::parameters resources
  won't come from the procps package itself, so the default priority of 10 is
  neither sensible nor safe.
* Change priority of Sysctl::Parameters['lvs'] to 60 so that it conforms with
  the priority namespacing policy of the package. (The 50 range is not
  defined.)

Change-Id: If004835fb369810bba553dce2d2b8df7b8d364ac
Merge "Add Icinga checks for important sysctl params" into production