Run and add ssh key to agent (#6)

After the key is generated, automatically add it to the ssh-agent by ssh-add `~/.ssh/cscs-key`.
It used have to edit the `~/.ssh/config` to point to the `cscs-key`. If the ssh-agent contains the key, then the ssh-config file can kept simple and works charm with the new `ComputationalResourceWidget`.

start.py

@@ -96,8 +96,20 @@ def setup(self, _=None):
             )
             return
         self.store_the_keys(*keys)
+
+        # Add the key to the ssh-agent.
+        self.add_key_to_ssh_agent()
+
         self.output.value = "The keys were updated 👍"
 
+    def add_key_to_ssh_agent(self):
+        """Add the key to the ssh-agent."""
+        subprocess.run(
+            ["ssh-add", "-t", "1d", str(self.private_key_file)],
+            encoding="utf-8",
+            check=True,
+        )
+
     def get_keys(self):
         headers = {"Content-Type": "application/json", "Accept": "application/json"}