CLI option for number of CPUs when creating a new boot2docker VM

[zarko-tg]

@BOSS :)

Question / enhancement:

I suppose there's no current option to specify number of CPUs with init on command line.
Would be great to use it in a similar fashion as -m, --memory

Is it possible at all?
Otherwise, any reason to worry about the VirtualBox warning?

[tianon]

+1 noticed this myself the other day; my system has 8 cores, but my
boot2docker VM doesn't need that many :)

[SvenDowideit]

+1 - feel free to make a PR - that'll be faster than waiting for us :)