Migrate context list to config storage

[mkuziemko]

Description

Changes proposed in this pull request:

• remove ListHubServer which used Keys method. Migrate to config storage. (more context here)

Tests

1. Get the list of contexts:

   $ ./bin/capact-linux-amd64 config get-contexts
   SERVER   AUTH TYPE   DEFAULT  
   ---------+-----------+----------

2. Login to sample server:

   $ ./bin/capact-linux-amd64 login
   ? Gateway's server address:  localhost:8080
   ? Username:  test
   ? Password:  ****
   Login Succeeded

3. Repeat step (1)

         SERVER           AUTH TYPE    DEFAULT  
   -------------------------+------------+----------
   https://localhost:8080   Basic Auth   NO  

4. Repeat step (2) and (3)

   
   $ ./bin/capact-linux-amd64 login
   ? Gateway's server address:  test.com
   ? Username:  test
   ? Password:  ****
   Login Succeeded

$ ./bin/capact-linux-amd64 config get-contexts SERVER AUTH TYPE DEFAULT
-------------------------+------------+---------- https://localhost:8080 Basic Auth YES
https://test.com Basic Auth NO

5. Verify if the contexts exists in created config file:

availablecontexts:

• https://localhost:8080
• https://test.com ...

  
  ## Related issue(s)

• https://github.com/capactio/capact/issues/405