Deprecate num-instances in favor of min/max count

For 'nova boot', num_instances only set max_count, which will try to
launch up to max_count instances. But sometimes users want to either
launch min_count instances or nothing. So deprecate --num-instances
and use --min-count and --max-count instead.

DocImpact: deprecate '--num-instances' for 'nova boot'

Change-Id: If0088aef554a528258799b22c6e793541474cb0d
Closes-bug: #1309244
Merge "Deprecate num-instances in favor of min/max count"