Wait for SSH command to complete after execution

This patch resolves a problem that started to show up when fast commands,
such as 'cat', were executed on target. The original code was killing
the command once we faced a return_value == None. This is obviously wrong
and it probably comes from multiple SSH module implementations.

The right thing to do is to wait for the process completion, once we
finish to read stdout. In this way, return_value will be != None and we
can process fast commands execution.

libkirk/ssh.py

@@ -269,8 +269,7 @@ async def run_command(
                 if proc:
                     self._procs.remove(proc)
 
-                    if proc.returncode is None:
-                        proc.kill()
+                    await proc.wait()
 
                     ret = {
                         "command": command,