Lazy load the cli.Client machine

[rochacbruno]

Problem:

Everytime we instantiate a new instance of cli.Client its __init__ method
code tries to make communication with the remote host to determine if it is
a local or ssh machine to be used.

The communication with machine should be done only when it is necessary,
which is when the run command is called.

To reproduce the problem

{
  "hosts": [
    {
      "hostname": "this-host-does-not-exists-or-server-is-down",
     ...
    }
  ]
}

$ export PULP_SMASH_CONFIG_FILE=/path/to/above/settings.json
$ python -c "from pulp_smash import cli, config; c = cli.Client(config.get_config()); print(c)"
Traceback (most recent call last):
plumbum.machines.session.SSHCommsError: No communication channel detected. Does the remote exist?

We have not called the run method, so the communication with server should not be done yet.

Solution:

Transform machine in a lazy @property then it is instantiated only
when it is first called.

Not calling run

$ python -c "from pulp_smash import cli, config; c = cli.Client(config.get_config()); print(c)"
<pulp_smash.cli.Client object at 0x7f5439275ef0>

When run is called

$ python -c "from pulp_smash import cli, config; c = cli.Client(config.get_config()); c.run(['ls'])"
Traceback (most recent call last):
plumbum.machines.session.SSHCommsError: No communication channel detected. Does the remote exist?

[ragabala]

@rochacbruno thanks for the documentation 💯