Dell S6000: Fix reboot failure issue

[aravindmani-1]

- Why I did it
Reboot command in Dell S6000 failed to reboot the switch.
- How I did it
Added retry mechanism and CPU reset.
- How to verify it
Execute reboot command in Dell S6000.
- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012

- Description for the changelog
S6000_reboot_UT.txt

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

[jleveque]

@aravindmani-1: You flagged that this patch should be backported to the 201811 branch. Therefore, I assume it should be backported to the 201911, 202006 and 202012 branches as well, no?

[aravindmani-1]

Hi @jleveque , Changed the cherry pick request to other branches.
Will raise a separate PR for 201811 branch.

[abdosi]

@aravindmani-1 Please create PR for 201911.

[abdosi]

Please create PR.

@rlhui

[aravindmani-1]

@abdosi We will create a PR. #6835 will override this PR's changes.
So, we will raise the PR accordingly.

[abdosi]

do we need this in 201911 ? #6835 had label removed for 201911