Move nova.utils.cpu_count() to processutils module

This moves and renames nova.utils.cpu_count() utility method from nova
to oslo.  Added in commit 75c96a48fc7e5dfb59d8258142b01422f81b0253, this
is used to set the default number of nova api/conductor workers if not
explicitly specified in nova.conf.

I'm moving this to oslo so that we can use the same default worker logic
in other projects, i.e. cinder, trove, glance and taskflow.

Closes-Bug: #1333370

Change-Id: I81748a15f8da85e287afdb5ec9de6369f8919ab0
Merge "Move nova.utils.cpu_count() to processutils module"