Add retry decorator to SSH "execute" method

In case of SSH timeout (TimeoutException, TimeoutError), the
tenacity.retry decorator retries the execution of the SSH
"execute" method up to 10 times.

Some SSH execute calls, related to QoS scenario tests, have been
enhanced by setting a relatively small timeout value. The commands
executed should be quick enough to be executed in this amount of time.
In case of timeout (due to communication problems), the retry decorator
will send again the command to be executed.

Change-Id: Idc0d55b776f499a4bc5d8c9d9a549f0af8f3fac0
Closes-Bug: #1844516
diff --git a/requirements.txt b/requirements.txt
index bb836d1..2febb7e 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -13,6 +13,7 @@
 paramiko>=2.0.0 # LGPLv2.1+
 six>=1.10.0 # MIT
 tempest>=17.1.0 # Apache-2.0
+tenacity>=3.2.1 # Apache-2.0
 ddt>=1.0.1 # MIT
 testtools>=2.2.0 # MIT
 testscenarios>=0.4 # Apache-2.0/BSD