cpu plugin: add microcode revision

[m4rcu5]

Description

In light of the spectre_v1,spectre_v2,spec_store_bypass vulnerabilities that got attention in recent months. It would be nice to have the microcode revision exported in the cpu tree as well. The information is avaiable in the the /proc/cpuinfo file which is used as source for the cpu tree here ohai/lib/ohai/plugins/linux/cpu.rb.

~ % grep -E 'processor|model name|microcode' /proc/cpuinfo 
processor	: 0
model name	: Intel(R) Core(TM) i5-4590 CPU @ 3.30GHz
microcode	: 0x25
processor	: 1
model name	: Intel(R) Core(TM) i5-4590 CPU @ 3.30GHz
microcode	: 0x25
processor	: 2
model name	: Intel(R) Core(TM) i5-4590 CPU @ 3.30GHz
microcode	: 0x25
processor	: 3
model name	: Intel(R) Core(TM) i5-4590 CPU @ 3.30GHz
microcode	: 0x25