Allow more SQS settings to be set by env var

[mlarraz]

Add a few optional settings that can be configured by environment variable:

• SCOUT_SQS_MAX_NUMBER_OF_MESSAGES - Max number of SQS messages to fetch at once
• SCOUT_SQS_WAIT_TIME_SECONDS - Max seconds to wait for an SQS message per poll
• SCOUT_SQS_VISIBILITY_TIMEOUT - How long to hide an SQS message after receiving it

These can also be set in the YAML config under a new sqs key:

sqs:
  max_number_of_messages: 10
  wait_time_seconds: 30
  visibility_timeout: 30

[rnubel]

Since the SQS client is really owned by Queue, I think it'd be cleaner to add these properties onto the Config object and let Queue handle passing them down to SqsClient at the time it initializes it. Passing them under-the-table with this global variable is breaking the encapsulation.

I don't have a strong opinion on if these need to come from the config file as opposed to coming in from ENV vars, but if you add these sqsSettings into the config object (e.g. config.SQS), then I think we'll get the ability to set them from the config file for free which will fit better into the way that Enova uses Scout.

[mlarraz]

Let me know if the updated commit does what you're thinking. Ideally I want to make the config file entirely optional and allow everything to be set via environment

[rnubel]

Sorry about the delay here. I've set up email notifications now so I should be more responsive now.

Clever trick with the embedding! I added suggestions for yaml tags so the values could be parsed out of the config in a uniform fashion -- if that breaks the embedding, though, we can move forward without them.