arg_validator: support generating default value for ArgValidatorDict
arg_validator: support generating default value for ArgValidatorDict

Some dictionaries may wish to have a default value that is a dictionary
with the default of all keys.

Add a method to generate this.

This returns a callable instead of a plain dictionary. In practice, the
generated default value should always be the same. However, we also
expect that ArgValidator validation gives a result that is owned by the
caller. In case of mutable values (like a dictionary), we want a deep
clone.

Signed-off-by: Thomas Haller <16e26612e4159990a6b3691e92592e691ff74132@redhat.com>

linux-system-roles/network,linux-system-roles/network,linux-system-roles/network