selftest: Add helper function to avoid repeated {DC_SERVER}=x

selftest: Add helper function to avoid repeated {DC_SERVER}=x

For every 2-DC testenv we setup, we copy the $DC_SERVER value of the
dependent testenv (i.e. the PDC) into the env variables for the new
testenv. This means DC_SERVER always points to the PDC (or first DC).

This adds a helper function to avoid repeating this code for every 2-DC
environment we setup.

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