Add a directory-specific nameserver data to templates

Add a directory-specific nameserver data to templates

If a template is being rendered into a directory that represents a
nameserver (e.g. "ns1"), include a nameserver-specific information in
the data - variable called "ns" which has information about the
nameserver this file belongs to.

Ensure the "ns" variable is only exposed to the template when rendered,
without affecting the environment variables (always work with a copy of
the env_vars).

(cherry picked from commit aa435b2e036a668d87579495cae1e31524a97f31)