avocado.utils.process: Move subprocess initialization to public method calls

Currently, as soon as we initialize a process.SubProcess
object, a subprocess.Popen() object is created. This breaks
the user's expectation that the process will start when
run() is called. Therefore, move process execution to
a private method, and call that method in all public
methods of the class.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.utils.process: SubProcess() -> Add .start() method

Now that we do not initialize the subprocess upon class
initialization, add a utility method .start(), that will
start running the command, useful for background commands.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>