Fix CDP failure in make test

Once the hostname is longer than 15 bytes,
the name of device in the output of show cdp command will be truncated,
and CDP test case will fail with below message.

==============================================================================
FAIL: test_send_cdp_packet (test_cdp.TestCDP)
------------------------------------------------------------------------------
Traceback (most recent call last):
File "vpp/test/test_cdp.py", line 92, in test_send_cdp_packet
    "CDP received invalid device id")
File "vpp/test/framework.py", line 804, in assert_equal
    self.assertEqual(real_value, expected_value, msg)
AssertionError: Invalid CDP received invalid device id: net-x86-supermi does not match expected value net-x86-supermicro-02

Change-Id: Ia32b92c6cd1bb6070adcee3ec45e38399ec382a7
Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>