Specify Custom Basic Auth password for Agents when adding Node

pmm-admin's config and register commands should be extended with a new flag to support a custom password for exporters' metrics auth:

#pmm-admin register --agent-password=AGENT-PASSWORD
#pmm-admin config --agent-password=AGENT-PASSWORD

pmm-agent's setup command should be extended with a new optional flag --agent-password

Testing

Prerequisities:

1. pmm-server is up and running

2. local pmm-agent is registered and running (for cases C1 and C2)

Check routines: 

Verify exporter is up and running (user/password)

1. navigate to localhost:<PORT>/metrics (find node exporter's <PORT> in logs while starting pmm-agent)

        browser should ask a user/password to access a page

        metrics page should open

2. navigate to <PMM_ADDR>/graph/d/node-instance-summary/node-summary

3. select your node from Node Name dropdown

        dashboard should display actual data for your local node_exporter

Cases:

C1. pmm-admin config

    1.1. run the command

./bin/pmm-admin config --agent-password=<PASSWORD>

        use --force flag to override existing node error if needed;

    1.2. verify new password is stored in pmm (i.e. in agents table)

    1.3. restart the agent

    1.4. [Verify exporter is up and running] (pmm/<PASSWORD>)

C2. pmm-admin register

    2.1. run the command:

./bin/pmm-admin register --agent-password=<PASSWORD>

        use --force flag to override existing node error if needed;

    2.2. verify new password is stored in pmm (i.e. in agents table)

    2.3. restart the agent

    2.4. [Verify exporter is up and running] (pmm/<PASSWORD>)

C3. pmm-agent setup

    3.1. run the command: 

pmm-agent setup --config-file=pmm-agent-dev.yaml --server-address=<PMM_ADDR>:<PORT> --agent-password=<PASSWORD>

        use --force flag to override existing node error if needed;

        use --paths-exporters_base flag to specify binaries location if needed;

        if TLS is disabled - authenticate via user/password:

        --server-insecure-tls --server-username=<PMM_USER> --server-password=<PMM_PASS>

    3.2. verify new password is stored in pmm (i.e. in agents table)

    3.3. start the agent

    3.4. [Verify exporter is up and running] (pmm/<PASSWORD>)

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Original description: In PMM-7570, the node exporter is not receiving the custom password, it still uses agent ID.