refactor connect, login and register flows

[miskun]

In order to streamline the usage of Kontena, following enhancements should be made:

1. kontena register should be changed so that it is possible to register at any time. The command without any arguments should register user to Kontena's default and public auth provider service at address https://auth.kontena.io. User may provide address to some other auth provider service as an argument for this command.
2. Master Node should have configurable parameter for defining the auth provider address. Any login attempts to Master Node should be authenticated via this auth provider. By default, this address should be Kontena's default and public auth provider service at address https://auth.kontena.io.
3. kontena login should be changed so that it is requiring an argument describing the Master Node address. Show Kontena ASCII art during login :)
4. Get rid of kontena connect as there is no more need for it.