Get rid of ohai

Ohai required support additional packages,
and unfortunatelly, not all of them are
opensource friendly (ruby-sigar, for example).

This changes will let us to rid ruby-mixlib*,
ruby-sigar and ruby-yajl packages.

Also, it may sound strange, but ohai[:virtualization]
makes decision based on /proc/cpuinfo information
only (this applies only to kvm/qemu, other virt-systems
determines correctly, AFAICS).

So, if someone will choose a non-default (qemu)
processor configuration, ohai will return incomplete
information about virtualization on a kvm-based virtual host.