added route_mode support for NLB vnf

[ujjwal-ibm]

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Relates OR Closes #0000

Output from acceptance testing:

=== RUN   TestAccIBMISLB_basic_network_vnf
--- PASS: TestAccIBMISLB_basic_network_vnf (1510.79s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm 1513.447s

=== RUN   TestAccIBMISLBDatasource_basic
--- PASS: TestAccIBMISLBDatasource_basic (433.70s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm 436.315s

=== RUN   TestAccIBMISLBSDatasource_basic
--- PASS: TestAccIBMISLBSDatasource_basic (516.82s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm 520.342s

[hkantare]

Please share the testresults of all testcases for changed resources/datasources