feat: simplify work with Database Lab CLI (port forwarding) (platform#91)

Description

Simplify work with Database Lab CLI (port forwarding)

Related issue

platform#91

Examples

Command line:

$ dblab port-forward
2020/06/17 17:48:40 [INFO]   The connection is available by address: 127.0.0.1:3000


$ dblab clone port-forward CLONE_ID
2020/06/17 17:54:27 [INFO]   The clone is available by address: 127.0.0.1:3000


$ dblab --forwarding-local-port 6000 clone port-forward CLONE_ID
2020/06/17 17:57:33 [INFO]   The clone is available by address: 127.0.0.1:6000


Initialization:
$ dblab init --environment-id stage \
  --url "http://127.0.0.1:2345" \
  --token "SECRET_TOKEN" \
  --forwarding-server-url "ssh://user@stage.srv:22" \
  --forwarding-local-port 8080


$ dblab config list
Available CLI environments:
ENV            URL                         Forwarding server URL        Forwarding local port
[ ] dev        http://127.0.0.1:3000                                    
[*] stage      http://127.0.0.1:2345       ssh://user@stage.srv:22      8080
[ ] local      http://local.example.com                                 
[ ] original   http://127.0.0.1:3000       ssh://john@remote.host:22    2400

Database Lab configuration example:
```yaml
current_environment: dev
environments:
  dev:
    url: http://127.0.0.1:3000
    token: xxx
    insecure: false
    forwarding:
      server_url: "ssh://user@1.1.1.1:22"
      local_port: "9000"

Checklist

• the MR description has been reviewed
• this MR contains text changes and they have been reviewed OR there are no texts changes
• this MR contains GUI/CLI changes and they have been reviewed OR there are no GUI/CLI changes
• this MR contains API changes, specifications reflect those changes and they have been reviewed OR there are no API changes

Closes #91 (closed)