pmm-admin does not use 'listen-port' value from the agent config file

If pmm-agent is configured to use a port different from the default 7777, pmm-admin does not use the value from /usr/local/percona/pmm2/config/pmm-agent.yml file and tries to connect to the default port.

For example:

Fragment of the configuration file:

# Updated by `pmm-agent setup`.
---
id: /agent_id/c575d159-26ea-4a14-8b9e-039413dc7560
listen-port: 7778

Ports used by pmm-agent:

$ sudo netstat -tunlp|grep pmm-agent
tcp 0 0 127.0.0.1:7778 0.0.0.0:* LISTEN 6075/pmm-agent 
tcp 0 0 127.0.0.1:43510 0.0.0.0:* LISTEN 6075/pmm-agent

Attempt to check the agent status:

$ sudo pmm-admin status
Failed to get PMM Server parameters from local pmm-agent: Post http://127.0.0.1:7777/local/Status: EOF.
Please use --server-url flag to specify PMM Server URL.

 
How to test:

1. Run pmm-agent on different port than 7777 (for example 8000)
2. Download attached binary of pmm-admin.
3. Use downloaded pmm-admin 
4. Add global flag --pmm-agent-listen-port=8000 to your command. For example:

./bin/pmm-admin --pmm-agent-listen-port=8000 add postgresql --username=pmm-agent --password=pmm-agent-password --query-source=pgstatmonitor nameofpostgres

5. See that command worked.
 

Suggested solution: 

Add --pmm-agent-listen-port flag to pmm-admin