feat: Add --parallel arg for insights-run

[ryan-blakley]

Check all that apply:

• Have you followed the guidelines in our Contributing document, including the instructions about commit messages?
• Is this PR to correct an issue?
• Is this PR an enhancement?

Complete Description of Additions/Changes:

Added parallel execution of rules executed via insights-run. Updated some docstrings that were missing args.

[xiangce]

@ryan-blakley, per my test, I think there is a bug in the following original function:

insights-core/insights/core/dr.py

Lines 988 to 994 in 37a6a88

	def generate_incremental(components=None, broker=None):
	components = components or COMPONENTS[GROUPS.single]
	components = _determine_components(components)
	seed_broker = broker or Broker()
	for graph in get_subgraphs(components):
	broker = Broker(seed_broker)
	yield (graph, broker)

It should not generate a new broker when there is an existing one. e.g.

     seed_broker = broker or Broker()
     for graph in get_subgraphs(components):
-        broker = Broker(seed_broker)
-        yield (graph, broker)
+        yield (graph, seed_broker)

[ryan-blakley]

@ryan-blakley, per my test, I think there is a bug in the following original function:

insights-core/insights/core/dr.py

Lines 988 to 994 in 37a6a88

	def generate_incremental(components=None, broker=None):
	components = components or COMPONENTS[GROUPS.single]
	components = _determine_components(components)
	seed_broker = broker or Broker()
	for graph in get_subgraphs(components):
	broker = Broker(seed_broker)
	yield (graph, broker)

It should not generate a new broker when there is an existing one. e.g.

     seed_broker = broker or Broker()
     for graph in get_subgraphs(components):
-        broker = Broker(seed_broker)
-        yield (graph, broker)
+        yield (graph, seed_broker)

Hmm yeah I've never understood the reasoning behind the seed broker option in the Broker class. I can update that function to remove the seeding.