Kaiser login command is confusing

[Metallion]

I think the most natural expectation for a command named kaiser login is to get a shell on a container. It however just executes a command and to get the shell you have to run kaiser login sh.

I believe it's better to split this into two commands:

• kaiser exec to run commands.
• kaiser login to get a shell. This would be equivalent to running kaiser exec sh

While we're at it, we might want to add a parameter to specify which container we want to do this against. Currently it's hard coded to the app container and I this should remain the default but perhaps a parameter like the following is a good idea?

kaiser exec uptime #runs uptime on the app container
kaiser exec -c db uptime #runs uptime on the db container

kaiser login # gets us a shell on the app container
kaiser login -c db #gets us a shell on the db container