Add backend.ai exec command to execute a shell command in a running session

[achimnol]

Currently backend.ai run command requires to specify a file to execute a shell command, due to a prior parsing limitation of Click with mixing required positional arguments and a zero-or-more positional argument.
This makes unnecessary confusion for new users and we feel also difficult to explain to them.

Let's make a new command, just like docker exec:

• backend.ai [root-options] exec {session-id-or-name} {cmdargs...}
  • Following the new CLI command hierarchy (refactor: Make CLI commands consistent backend.ai-client-py#163), backend.ai exec should be an alias of backend.ai session exec.
  • The session ID or name may be a partial prefix of session ID or names, like in backend.ai rm.
  • This command does not create the session but raises an explicit error if it does not exist.
  • All the rest of arguments are passed as-is.

Considerations for implementing this feature in various client environments:

Client Env	Command/Function	Implementation detail
CLI	backend.ai run --exec "<cmdargs>" <image> <files>	session create API + kernel file upload API + batch-mode execution API
CLI	backend.ai app <session> <appname>	stream proxy API
CLI	backend.ai ssh <session> <cmdargs>	kernel file download API (to get ssh key) + alias of app command + wrapper of OpenSSH client
CLI	backend.ai exec <session> <cmdargs>	alias of ssh command
Browser	Running a shell command in session	batch-mode execution API

The advantage of making exec to be an alias of ssh is that it supports the full terminal and interactive stdin support through the SSH session. But in the web browsers we don't have the SSH client available for Javascript codes, so we need to fallback to use the batch-mode execution API.