tests: Tweak DRS tests to avoid unnecessary env variables

tests: Tweak DRS tests to avoid unnecessary env variables

Each DC used in a DRS test has its own '<testenv>_SERVER' environment
variable, e.g. VAMPIRE_DC_SERVER. These variables are only used by
test.py for DRS, but they're not actually needed.

The $SERVER environment variable holds the same information, so we can
just use this in test.py instead.

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