Added salt-ssh integration test for custom module within SLS

This PR reproduce the issue mention in #16271

As of aa6f87e62, this integration test is giving the following output

```
$ python2 tests/runtests.py -C --ssh
. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 * Transplanting configuration files to '/tmp/salt-tests-tmpdir/config'
 * Current Directory: /tmp/salt2
 * Test suite is running under PID 1750
 * Logging tests on /tmp/salt-runtests.log
. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Setting up Salt daemons to execute tests
.  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 * Waiting at most 0:02:00 for minions(minion, sub_minion) to connect back
   * minion connected.                                                                                                                                   
   * sub_minion connected.                                                                                                                               
 * Syncing minion's modules (saltutil.sync_modules)
   * Synced minion modules: modules.override_test, modules.runtests_decorators, modules.runtests_helpers, modules.salttest                               
   * Synced sub_minion modules: modules.override_test, modules.runtests_decorators, modules.runtests_helpers, modules.salttest
 * Syncing minion's states (saltutil.sync_states)
   * Synced minion states: states.salttest                                                                                                               
   * Synced sub_minion states: states.salttest
 * Initializing SSH subsystem
.  ===============================================================================
.  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Starting CLI Tests
.  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
....[ERROR   ] Git-based ext_pillar is enabled in configuration but could not be loaded, is GitPython installed?
[CRITICAL] Specified ext_pillar interface git is unavailable
[CRITICAL] Specified ext_pillar interface git is unavailable
[CRITICAL] Specified ext_pillar interface test_ext_pillar_opts is unavailable
F
.  =====================================================================
FAIL: test_ssh_sls_with_custom_module (integration.cli.custom_module.SSHCustomModuleTest)
.  ----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/salt2/tests/integration/cli/custom_module.py", line 71, in test_ssh_sls_with_custom_module
    raise AssertionError(cmd[key]['comment'])
AssertionError: Module function test.recho is not available

.  ----------------------------------------------------------------------
Ran 5 tests in 38.387s

FAILED (failures=1)

.  ==============================  Overall Tests Report  ================================
*** CLI Tests      *****************************************************************************************************
. --------  Failed Tests   --------------------------------------------------------------------------------------------------
   -> integration.cli.custom_module.SSHCustomModuleTest.test_ssh_sls_with_custom_module    ................
       Traceback (most recent call last):
         File "/tmp/salt2/tests/integration/cli/custom_module.py", line 71, in test_ssh_sls_with_custom_module
           raise AssertionError(cmd[key]['comment'])
       AssertionError: Module function test.recho is not available
.   ......................................................................................................................................................
.  ----------------------------------------------------------------------------------------------------------------------------
.  ==============================================================================
FAILED (total=5, skipped=0, passed=4, failures=1, errors=0) 
.  ================================  Overall Tests Report  ==============================
```