Parallels: Use "memory" and "cpus" customization shortcuts

[legal90]

memory and cpus customization parameters are widely used in .kitchen.yml of different community cookbooks.
But currently it doesn't work for "parallels" provider, because prlctl set command doesn't support "memory" argument (it expects to get a "memsize" instead).

However, this provider actually supports memory and cpus as customization shortcuts (it was implemented for similarity with virtualbox provider):

config.vm.provider "parallels" do |prl|
  prl.memory = 1024
  prl.cpus = 2
end

So, this PR makes it possible to use memory and cpus for "parallels" provider.

cc: @fnichol

[legal90]

I've just fixed unit test and added a new test case

[cheeseplus]

+1