Make machine name optional in CLI

For commands inspect, ip, kill and restart.
It is already implemented for the others.

Signed-off-by: Guillaume Giamarchi <guillaume.giamarchi@gmail.com>
Avoid code duplication

Signed-off-by: Guillaume Giamarchi <guillaume.giamarchi@gmail.com>
Merge pull request #103 from ggiamarchi/machine-name-optional

Make machine name optional in CLI