fix ipv6 only issue

[yabinma]

Describe your changes

Provide Issue ticket number if applicable/not in title

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Checklist before requesting a review

• My changes affect only 10 files or less.
• I have performed a self-review of my code and tested it.
• If it is a new feature, I have added thorough tests, my code is <= 1450 lines.
• If it is a bugfix, my code is <= 200 lines.
• My functions are <= 80 lines.
• I have had my code reviewed by a peer.
• My unit tests pass locally.
• Netmaker is awesome.

[yabinma]

@abhishek9686 , please help review. This is the fix for the issue mentioned today. If creating a ipv6 only netmaker network, it could not add host properly, and the hosts could not communicate each other properly.

After the fix, the ipv6 network can be created successfully and all the hosts added into this network work well.