[config db] Trim garbage charactor in "DEVICE_METADATA" of config db

[wenyiz2021]

This is to prevent a user error in ADO: 28004860 where there was an extra "\n" at the end of mac, in "DEVICE_METADATA" section.

What I did

1. the garbage char could be generated with "load_minigraph" if '-override-config' is specified, and if golden_config has garbage char.
2. it could also be introduced by "config reload"

How I did it

in both cases, add the strip method for "mac" as well as "platform" whenever the DEVICE_METADATA tends to be modified. to trim the extra "\n"

How to verify it

1. add an extra "\n" at end of mac, in config db
2. did config reload, garbage char exists
3. apply changes in this PR

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)