Add kickstart.user functions

docs/kickstart/print_with_separator.md

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+# kickstart.print_with_separator separator [arguments...]
+
+Print the list of `arguments` separated by `separator`
+
+
+### Example
+
+```bash
+$ kickstart.print_with_separator , a b c
+a,b,c
+
+$ kickstart.print_with_separator '|' a b c
+a|b|c
+```

docs/kickstart/user/is_on_group.md

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+# kickstart.user.is_on_group user group
+Return 0 if `user` is on `group`, and 1 if not.
+
+### Example
+
+```bash
+$ kickstart.user.is_on_group vagrant vagrant && echo yup
+yup
+
+$ kickstart.user.is_on_group vagrant no_group || echo nope
+nope
+```

docs/kickstart/user/remove_group.md

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+# kickstart.user.remove_group user group
+Check if `user` is part of `group` and remove it from `group`
+
+### Example
+
+```bash
+$ kickstart.user.remove_group vagrant admin
+```