Allow user to specify 'listen address' to pmm-agent otherwise default to 127.0.0.1

The problem: it's hard to implement good monitoring in DBaaS with pmm-agent listening on 127.0.0.1 only.

Proposed solution: allow to specify alternative listening address, including "all addresses" option.

We would like to use liveness probes for pmm-agent like that one:

livenessProbe:
  httpGet:
    port: 7777
    path: /local/Status
  timeoutSeconds: 2
  initialDelaySeconds: 30
  periodSeconds: 10
  failureThreshold: 3

But that's not currently possible due to pmm-agent listening on 127.0.0.1, not on all network interfaces.

There should be a flag / configuration field / environment variable --listen-address / listen-address / PMM_AGENT_LISTEN_ADDRESS to allow one to specify network interface to listen to, or "0.0.0.0" to listen on all network interfaces. "127.0.0.1" should stay as a default value.

To QA:

• without this flag, pmm-agent should work as before, and local API should be available only on 127.0.0.1;
• with this flag, pmm-agent's local API should be available on that IP address;
  • in particular, using 0.0.0.0 should make this API available on all IP addresses.