[SSH] Fix SSH login session limit test case randomly failed issue. #5426 (#5516)

### Description of PR
    Fix SSH login session limit test case randomly failed issue. The issue is because update config file with 'tee' command, and tee command will exit when receive pipeline close command.

### Type of change
- [ ] Bug fix
- [ ] Testbed and Framework(new/improvement)
- [x] Test case(new/improvement)


### Back port request

### Approach
#### What is the motivation for this PR?
   Fix SSH login session limit test case randomly failed issue. 

#### How did you do it?
   Code change:
         1. Not use tee command to update template file in template folder.
         2. Create template file with pipeline in home folder and move to template folder.

#### How did you verify/test it?
   Run new UT make sure they are all pass.
   Make sure all current UT not break during merge validation.

#### Any platform specific information?
   N/A

#### Supported testbed topology if it's a new test case?

Author: liuh-80

tests/ssh/test_ssh_limit.py

@@ -26,7 +26,8 @@
 
 PAM_LIMITS_TEMPLATE_PATH = "/usr/share/sonic/templates/pam_limits.j2"
 LIMITS_CONF_TEMPLATE_PATH = "/usr/share/sonic/templates/limits.conf.j2"
-LIMITS_CONF_TEMPLATE = "echo \"{{% if hwsku == '{0}' and type == '{1}'%}}\n{2}\n{{% endif %}}\"  | sudo tee {3}"
+LIMITS_CONF_TEMPLATE_TO_HOME = "echo \"{{% if hwsku == '{0}' and type == '{1}'%}}\n{2}\n{{% endif %}}\"  > ~/temp_config_file"
+TEMPLATE_MOVE_COMMAND = "sudo mv ~/temp_config_file {0}"
 
 def get_device_type(duthost):
     device_config_db = json.loads(duthost.shell("sonic-cfggen -d --print-data")['stdout'])
@@ -42,7 +43,12 @@ def get_device_type(duthost):
 def modify_template(admin_session, template_path, additional_content, hwsku, type):
     admin_session.exec_command(TEMPLATE_BACKUP_COMMAND.format(template_path))
     admin_session.exec_command(TEMPLATE_CREATE_COMMAND.format(template_path))
-    admin_session.exec_command(LIMITS_CONF_TEMPLATE.format(hwsku, type, additional_content, template_path))
+    admin_session.exec_command(LIMITS_CONF_TEMPLATE_TO_HOME.format(hwsku, type, additional_content))
+    admin_session.exec_command(TEMPLATE_MOVE_COMMAND.format(template_path))
+
+    stdin, stdout, stderr = admin_session.exec_command('sudo cat {0}'.format(template_path))
+    config_file_content = stdout.readlines()
+    logging.info("Updated template file: {0}".format(config_file_content))
 
 def modify_templates(duthost, tacacs_creds, creds):
     dut_ip = duthost.mgmt_ip
@@ -86,10 +92,6 @@ def setup_limit(duthosts, rand_one_dut_hostname, tacacs_creds, creds):
         modify_templates(duthost, tacacs_creds, creds)
         restart_hostcfgd(duthost)
 
-        # for debug, print rendered config file
-        config_file_content = duthost.shell('sudo cat /etc/security/limits.conf', module_ignore_errors=False)['stdout_lines']
-        logging.debug("Updated config file: {0}".format(config_file_content))
-
     yield
 
     if template_file_exist: