Add plugin from monitoring systemd unit/target and its dependencies

[tomaszdurka]

• Plugin configuration should allow to define a list of units/targets to monitor (similar like log-parser config).
• Each monitored unit should create 1 metric, which has value 0 or 1, indicating whether all dependent units are active.

[tomaszdurka]

@njam or @kris-lab please review

I have one failing test on ruby 1.9.3. Seems there is a problem with detecting the right encoding for the file. Google suggests to add # encoding: utf-8 on top of the file (somwhere?).

[tomaszdurka]

Utf-8 characters problem solved.

[njam]

Reviewing

[njam]

• systemd_unit.rb: use https://github.com/cargomedia/komenda to get rid of boilerplate code
  • Pass commands with arguments as array, to have proper shell escaping
• systemd_unit.rb: monitor: Define and return Hash inline on last line, instead of creating it, then modifying and finally returning it?
• systemd_unit.rb: monitor: Afaik we don't have any boolean metrics so far do we? Usually we use 0 or 1. Did you verify that it works? If not, then let's use integer.
• systemd_unit.rb: Move non-public functions to a "private" block
• systemd_unit.rb: units: Rename to unit_dependencies to clarify purpose?
• systemd_unit.rb: units: I would simplify the external command called (only systemctl), and move the output cleaning to Ruby
• Update docu (readme.md, services.md)
• systemd_unit_spec.rb: To increase the coverage how about mocking Komenda.run?