[test_fg_ecmp] Fix 'DEFAULT_NAMESPACE' is not defined

[JibinBao]

Description of PR

In ecmp/test_fgnhg.py , 'DEFAULT_NAMESPACE' is not defined：

def fg_ecmp(ptfhost, duthost, router_mac, net_ports, port_list, ip_to_port, bank_0_port, bank_1_port, prefix_list): 



      # Init base test params 

      if isinstance(ipaddress.ip_network(prefix_list[0]), ipaddress.IPv4Network): 

          ipcmd = "ip route" 

      else: 

          ipcmd = "ipv6 route" 



      vtysh_base_cmd = "vtysh -c 'configure terminal'" 

>       vtysh_base_cmd = duthost.get_vtysh_cmd_for_namespace(vtysh_base_cmd, DEFAULT_NAMESPACE) 

E       NameError: global name 'DEFAULT_NAMESPACE' is not defined 

Summary:
Fixes # (#3323)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Approach

What is the motivation for this PR?

Fix the issue that 'DEFAULT_NAMESPACE' is not defined

How did you do it?

Import using following line:
from tests.common.helpers.constants import DEFAULT_NAMESPACE

How did you verify/test it?

Run test_fg_ecmpt test case

Any platform specific information?

no

Supported testbed topology if it's a new test case?

no

Documentation

[liat-grozovik]

/azp run

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