[ptf] Fixed root ssh authentication issue

[nazariig]

Signed-off-by: Nazarii Hnydyn nazariig@mellanox.com

SSH authentication via root is a common issue:
without this fix all PTF tests fails with the latest PFT docker.

- What I did

• Fixed root ssh authentication issue

- How I did it

• Made sed rules more robust

- How to verify it

1. make configure PLATFORM=mellanox
2. make target/docker-ptf-mlnx.gz

- Description for the changelog

• N/A

- A picture of a cute animal (not mandatory but encouraged)

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[nazariig]

@pavel-shirshov please have a look

[lguohan]

retest vmimage please

[lguohan]

retest vsimage please

[lguohan]

retest vs please