Add tests for string values for permit_root_login
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Add a check for allowed permit_root_login values

The permit_root_login setting is designed to reflect the possible settings
in the sshd_config (yes, no, without-password, forced-commands-only). Since
'true' and 'false' do not work and there seems no easy way to support both
type of values, the module now checks for valid values of the setting.
Increase module version