Use (# of CPUs) workers by default

Update the default value of the worker configuration options to be
equal to the number of CPUs on the system.  This is the most common
value that should be used to get ideal performance from nova-conductor
or nova-api.

The following options are updated in this change:

    [default]
    ec2_workers=
    osapi_compute_workers=
    metadata_workers=

    [conductor]
    workers=

DocImpact
UpgradeImpact

Change-Id: Id8009b5fd37cef03ca66cb5bf44211039ac895ba
Merge "Use (# of CPUs) workers by default"