Allow to test process parameters

Any parameter that can be queried with `ps` can be used for tests. For
example:

```
describe process("memcached") do
  its(:args) { should match /-c 30000\b/ }
end
```

There is a commit needed in specinfra for this to work. Moreover, I had to define `to_ary` in `Process` class. I don't know how `Cgroup` class got to work without it.