Use single quotes in command string to prevent shell expansion of passwords

When salt-ssh asks for a password to deploy its key it fails if the password contains a '$' because the generated shell command expands the '$' sign and the following characters as shell variable.

Using single quotes prevents shell expansion and passwords containig '$' signs work again.
Merge pull request #8307 from rsommer/patch-1

Use single quotes in cmd string to prevent shell expansion of passwords