Add timeout to retries when connecting

When connecting to SSH, we need to set a timeout in order to avoid
saturating OpenSSH server's connection pool. This can happen if SSH
starts listening to connections and the guest agent did not yet manage
to write the public keys for the user we are authenticating as.

coriolis/providers/backup_writers.py

@@ -252,16 +252,21 @@ class SSHBackupWriter(BaseBackupWriter):
             finally:
                 sftp.close()
 
-    @utils.retry_on_error()
+    @utils.retry_on_error(sleep_seconds=30)
     def _connect_ssh(self):
         LOG.info("Connecting to SSH host: %(ip)s:%(port)s" %
                  {"ip": self._ip, "port": self._port})
         ssh = paramiko.SSHClient()
         ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
-        ssh.connect(
-            hostname=self._ip,
-            port=self._port,
-            username=self._username,
-            pkey=self._pkey,
-            password=self._password)
+        try:
+            ssh.connect(
+                hostname=self._ip,
+                port=self._port,
+                username=self._username,
+                pkey=self._pkey,
+                password=self._password)
+        except:
+            # No need to log the error as we just raise
+            ssh.close()
+            raise
         return ssh