Fix Unicode issue in ipaddress for python2

[bingwang-ms]

Signed-off-by: bingwang [email protected]

What I did

This PR is to address the Unicode issue in fast-reboot-dump.py script in 201911.
There is another PR that addresses same issue in master image #1164
However, builtins package is not available in 201911 image.

Python 2.7.13 (default, Apr 16 2021, 14:02:03) 
[GCC 6.3.0 20170516] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import builtins
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named builtins

So, I create this PR for 201911 branch to fix the issue in another way.

How I did it

Add a custom method to convert string to unicode.

How to verify it

Verified on MSN2700, running SONiC.20191130.72.
First populate some FDB entry and ARP entry, and then run fast-reboot-dump.py script. No exception is raised, and we can confirm from syslog that all entries are dumped.

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)