Fix accessing instance attribute in AdvancedReboot

[vaibhavhd]

Description of PR

Fixes: Error in the tests using AdvancedReboot script

Type of change

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

Approach

What is the motivation for this PR?

Bug during accessing instance attribute in one of the methods of AdvancedReboot class.

How did you do it?

Use self to access the instance attribute creds to obtain the credentials.

How did you verify/test it?

Tested platform_tests/test_advanced_reboot.py::test_fast_reboot on T0 device and the test passed successfully.

Documentation

Snippet of the error that is being fixed:

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <tests.common.fixtures.advanced_reboot.AdvancedReboot instance at 0x7f6119dea3f8>

    def __buildTestbedData(self):
        '''
        Build testbed data that are needed by ptf advanced-reboot.ReloadTest class
        '''
        hostFacts = self.duthost.setup()['ansible_facts']
    
        self.mgFacts = self.duthost.minigraph_facts(host=self.duthost.hostname)['ansible_facts']
    
        self.rebootData['arista_vms'] = [
            attr['mgmt_addr'] for dev, attr in self.mgFacts['minigraph_devices'].items() if attr['hwsku'] == 'Arista-VM'
        ]
    
        self.hostMaxLen = len(self.rebootData['arista_vms']) - 1
        self.lagMemberCnt = len(self.mgFacts['minigraph_portchannels'].values()[0]['members'])
        self.vlanMaxCnt = len(self.mgFacts['minigraph_vlans'].values()[0]['members']) - 1
    
        self.rebootData['dut_hostname'] = self.mgFacts['minigraph_mgmt_interface']['addr']
        self.rebootData['dut_mac'] = hostFacts['ansible_Ethernet0']['macaddress']
        self.rebootData['vlan_ip_range'] = self.mgFacts['minigraph_vlan_interfaces'][0]['subnet']
        self.rebootData['dut_vlan_ip'] = self.mgFacts['minigraph_vlan_interfaces'][0]['addr']
    
>       self.rebootData['dut_username'] = creds['sonicadmin_user']
E       NameError: global name 'creds' is not defined

common/fixtures/advanced_reboot.py:125: NameError
========================== 1 failed in 63.12 seconds ===========================