tc-testing: don't hardcode 'ip' in nsPlugin.py

the following tdc test fails on Fedora:

 # ./tdc.py -e 2638
  -- ns/SubPlugin.__init__
 Test 2638: Add matchall and try to get it
 -----> prepare stage *** Could not execute: "$TC qdisc add dev $DEV1 clsact"
 -----> prepare stage *** Error message: "/bin/sh: ip: command not found"
 returncode 127; expected [0]
 -----> prepare stage *** Aborting test run.

Let nsPlugin.py use the 'IP' variable introduced with commit 92c1a19e
("tc-tests: added path to ip command in tdc"), so that the path to 'ip' is
correctly resolved to the value we have in tdc_config.py.

 # ./tdc.py -e 2638
  -- ns/SubPlugin.__init__
 Test 2638: Add matchall and try to get it
 All test results:
 1..1
 ok 1 2638 - Add matchall and try to get it

Fixes: 489ce2f4

 ("tc-testing: Restore original behaviour for namespaces in tdc")
Reported-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>