Let salt-ssh use 'platform-python' binary in RHEL8 (#191)

RHEL/CentOS 8 has an internal Python interpreter called 'platform-python'
included in the base setup.

Add this binary to the list of Python executables to look for when
creating the sh shim.

salt/client/ssh/__init__.py

@@ -147,7 +147,7 @@
 then SUDO="sudo "
 fi
 EX_PYTHON_INVALID={EX_THIN_PYTHON_INVALID}
-PYTHON_CMDS="python3 python27 python2.7 python26 python2.6 python2 python"
+PYTHON_CMDS="python3 /usr/libexec/platform-python python27 python2.7 python26 python2.6 python2 python"
 for py_cmd in $PYTHON_CMDS
 do
     if command -v "$py_cmd" >/dev/null 2>&1 && "$py_cmd" -c "import sys; sys.exit(not (sys.version_info >= (2, 6)));"