Make log routing for containers flexible

[megastef]

Making log routing more flexible:

• support multiple elasticsearch urls in LOGS_RECEIVER_URL container label/env var
• support multiple elasticsearch urls in logs-receiver-url Kubernetes annotation
• support list of pre-configured ES output plugins by setting LOGS_DESTINATION=configName,configName2

Example 1:
Run nginx and ship logs to local elasticsearch and to sematext cloud.

	docker run -d \
	-e LOGS_RECEIVER_URL="https://elasticsearch:9200/logs,https://logsene-receiver.sematext.com/YOUR_TOKEN_HERE" \
	-p 8282:80 nginx

Logagent will ship logs to both servers.

Example 2
Assume we have two elasticsearch output plugins configured in loggent.conf:

output: 
  semtatext:
    module: elasticsearch 
    url: https://logsene-receiver.sematext.com 
    index: YOUR_LOGS_TOKEN
  local-elk:
   module: elasticsearch 
   url: http://elasticsearch:9200
   index: logs

Then you can run a container and specify log destinations.

docker run -d -e LOGS_DESTINATION="sematext,local-elk" -p 8282:80 nginx

[otisg]

@megastef can you send the PR for docs, too?

[adnanrahic]

Only two things I noticed, the parseReceiverList functions can be refactored to use higher-order functions, and it should be put in a dedicated file so we can require it where it's needed to avoid code duplication.

[megastef]

@otisg yes, I'm working on the PR for Logagent docs.