'Push' metrics mode is default when adding services and nodes

User story: 

As a PMM user, I need all newly added Services and Nodes to be added in PMM with PUSH mode so that I don't need to carry about open ports anymore.  

UI:

"pmm-admin add ...."  will be adding with push mode by default (auto = push) 

Suggested implementation:

• return true for `isPushMode` function if metrics mode is push or auto.
  • https://github.com/percona/pmm-managed/blob/b055874a8e05363002de920d3cbb23aa51bba65e/services/management/management.go#L133
• Check if remote services added via UI still works in pull mode on pmm-server.
  • https://github.com/percona/pmm-managed/blob/PMM-2.0/services/agents/registry.go#L321 

How to test:

1. Run pmm-server

• All new exporters by default have push metrics mode.
  • Install latest pmm-client
  • register via pmm-admin config
    • check that node_exporter has push metrics mode.
  • pmm-admin add mysql
    • check that mysqld_exporter has push metrics mode.
  • pmm-admin add mongodb
    • check that mongodb_exporter has push metrics mode.
  • pmm-admin add postgresql
    • check that postgres_exporter has push metrics mode.
  • pmm-admin add proxysql
    • check that proxysql_exporter has push metrics mode.
  • pmm-admin add external
    • check that external exporter has push metrics mode.
• Services added with Older PMM clients ( with version 2.11.1 and older) still uses pull metrics mode and work well.
  • Install  pmm-client v 2.11.1 or older
  • register via pmm-admin config
    • check that node_exporter has pull metrics mode.
  • pmm-admin add mysql
    • check that mysqld_exporter has pull metrics mode.
  • pmm-admin add mongodb
    • check that mongodb_exporter has pull metrics mode.
  • pmm-admin add postgresql
    • check that postgres_exporter has pull metrics mode.
  • pmm-admin add proxysql
    • check that proxysql_exporter has pull metrics mode.
  • pmm-admin add external
    • check that external exporter has pull metrics mode.
• Services added via UI still works in pull mode.
  • Add AWS RDS MySQL Service via UI 
    • check that rds_exporter has pull metrics mode.
    • check that mysqld_exporter has pull metrics mode.
  • Add MySQL Service via UI
    • check that mysqld_exporter has pull metrics mode.
  • Add PostgreSQL Service via UI
    • check that postgres_exporter has pull metrics mode.
  • Add ProxySQL Service via UI
    • check that proxysql_exporter has pull metrics mode.
  • Add MongoDB Service via UI
    • check that mongodb_exporter has pull metrics mode.

DOC:

• Documentation updated to reflect the new default value
• all places where we mention "opening ports" updated, so they clear this is for PULL mode only as for now   
• documentation about push/pull usage is updated to reflect the version from which "push" is the default 

Acceptance criteria:

• All new exporters added by management API should work in push metrics mode.
  • Node exporter on Register endpoint
  • mysqld_exporter for AddMySQL endpoint
  • postgres_exporter for AddPostgreSQL
  • mongo_exporter for AddMongoDB
  • proxysql_exporter for AddProxySQL
  • External exporters
• Services added with Older PMM clients ( with version older than 2.11.1 ) still uses pull metrics mode and work well.
• Services added via UI still works in pull mode.