selftest: Use consistent env variables for dns_hub

selftest: Use consistent env variables for dns_hub

Setting up a testenv involves populating 2 different hashmaps - an
intermediary one (usually called 'ctx') and one that is used to populate
the testenv environment variables (usually called 'env_vars' or
'dcvars').
Because the dns_hub setup is very simple, it doesn't need two different
hashmaps. However, the variable names are still a mix of the two
hashmaps.

This patch updates dns_hub to use the second, more finalized hashmap
variable-names.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>