Added check to find accept_ra file exists

[tamilmani1989]

Reason for Change:

Incase if user disables ipv6 kernel setting, then accept_ra file would not exist and sysctl cmd would fail. This PR addresses that scenario

Issue Fixed:

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests

Notes:

[codecov]

Codecov Report

Merging #620 into master will increase coverage by 1.84%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #620      +/-   ##
==========================================
+ Coverage   37.44%   39.29%   +1.84%     
==========================================
  Files          47       43       -4     
  Lines        5215     4779     -436     
==========================================
- Hits         1953     1878      -75     
+ Misses       3000     2643     -357     
+ Partials      262      258       -4     

[pjohnst5]

lgtm

[andyliuliming]

this is a great change,
one of our customer are pending on this issue, they tried to disable the ipv6.
and then the azure cni can not work.

@tamilmani1989 could you please merge this and build a new image? thanks in advance.