Fix create relation bug

[shaopeng-lin]

Hi, I found a bug when creating a relation from cli.
`onos-cli-b987f85dd-snhxn:~$ onos topo create relation entity1 entity2
panic: runtime error: index out of range [2] with length 2

goroutine 1 [running]:
github.com/onosproject/onos-cli/pkg/topo.runCreateRelationCommand(0xc0004a4c00?, {0xc000470dc0, 0x2, 0x2?})
/build/pkg/topo/create.go:60 +0x99
github.com/spf13/cobra.(*Command).execute(0xc0004a4c00, {0xc000470d80, 0x2, 0x2})
/build/vendor/github.com/spf13/cobra/command.go:940 +0x862
github.com/spf13/cobra.(*Command).ExecuteC(0xc00049c000)
/build/vendor/github.com/spf13/cobra/command.go:1068 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
/build/vendor/github.com/spf13/cobra/command.go:992
github.com/onosproject/onos-cli/pkg/cli.Execute()
/build/pkg/cli/root.go:38 +0x1e
main.main()
/build/cmd/onos/main.go:12 +0x17`

And this pull request is a fix. Thanks.

[onf-cla-manager]

Hi @shaopeng-lin, this is the ONF bot 🤖 I'm glad you want to contribute to our projects! However, before accepting your contribution, we need to ask you to sign a Contributor License Agreement (CLA). You can do it online, it will take only a few minutes:

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[gab-arrobo]

@shaopeng-lin, are you planning to sign the CLA? If not, we might need to close your PR.