[cli] Added -a flag to the stop command

[rogkne]

Adds the -a flag in addition to the already existing --all flag to the sub command stop.

This provides a more consistent help output and it is handy for regular use:

Options:
  -h, --help         Displays help on commandline options
  -v, --verbose      Increase logging verbosity. Repeat the 'v' in the short
                     option for more detail. Maximum verbosity is obtained with
                     4 (or more) v's, i.e. -vvvv.
  -a, --all          Stop all instances
  -t, --time <time>  Time from now, in minutes, to delay shutdown of the
                     instance
  -c, --cancel       Cancel a pending delayed shutdown

[ricab]

Hi @rogkne, thank you for the PR and your interest in Multipass.

The absence of -a is intentional here. I know it is a little bit more effort, but the goal is to require a more conscious decision on the part of the user to affect all instances at once. And we want the flag to be consistent across commands (e.g. start, delete)

[rogkne]

Thanks @ricab for the clarification. I close the PR.