pmm-admin: Allow user-specified custom 'group' name when adding external services

Warning: group - Working name, might  change   before implementation

Goal: improve the User experience for adding  and viewing External services 

new Term:

External group - a custom name for the one or group of Services. specified by User based on his preferences  

Use story: 

As a PMM user, I need to be able to add External Service with specified external group so that I can easily see them on dashboards

 
Use case: 

The user has a cluster of Web servers with Mysql, Redis, and java.  Redis and Java have installed exporters. All servers in the cluster are the same. 
User want to have all this be monitored.  and to use one command on all servers. 

In his case, it  will be 

#pmm-admin add mysql
#pmm-admin add external --group=redis --listen-port=9121  

He wants this to be automated and added to his "automation" deployment script
He also expects this 2 external services will be presented on Node Dashboards as 2 separate different objects based on their external subtypes

UI
CLI
#pmm-admin add external --group=redis --listen-port=9121
GROUP - optional flag , default = "external"

Will add External service with service-name = $NODE_NAME-redis , 
Also,  user can set --service-name so some specific name  
We need to have an optional label in data  ="external_group=redis"   for better Dashboard presentation

Group should be case insensitive, convert to lower case.

Suggested implementation:

Add flag group to commands in pmm-admin.

1. pmm-admin inventory add service external.
   1. Flag
   2. Request to pmm-managed
   3. Message
2. pmm-admin add external 
   1. Flag
   2. Request to pmm-managed
   3. Message
3. pmm-admin list will expose external group as Type of the service like: "external:<external_group>"
4. pmm-admin inventory list services

How to demonstrate:

pmm-admin repo

Scenario: Inventory add empty subtype
    When: user run pmm-admin inventory add service external command with --group=<empty>
    Then: pmm-admin should set group=external by default and create external service

Scenario: Inventory add group matching existing type
    When: user run pmm-admin inventory add service external command with --group=redis/prometheus/e.t.c.
    Then: pmm-adming should set group provided by user and create external service

Scenario: Inventory add group not matching existing type
    When: user run pmm-admin inventory add command with --group=redis/prometheus
    Then: a new service should be added with group = group provided by user converted to lowercase

Scenario: pmm-admin list no extra flags 
    When: user run pmm-admin list with no extra flags
     And: there are external services with groups
    Then: external services type shown as "external:<external_group>"

 Scenario: pmm-admin list --json (if exists, this is only an example)
     When: user run pmm-admin list --json
      And: there are external services with groups
     Then: we should have two separate fields for `service_type` and for `group`