Add show reboot-cause history cli

[sujinmkang]

• What I did
  To make the reboot cause history information available for telemetry service, during processing the reboot-cause on boot, the information is saved in state-db.
  The cli command show reboot-cause is changed to support the new file format and file name.
  New command show reboot-cause history is added to display the reboot-cause information which is stored in state-db.

• How I did it
  Add the 'show reboot-cause history' command to read up to the10 previous reboot-cause information from state-db and display them.
  Read the last reboot-cause from the new file "previous-reboot-cause.json" and process the data to display for show reboot-cause

• How to verify it
  Unit test added.
  Verified also in DUT

• 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)

admin@sonic:~$ show reboot-cause history
name                 cause        time                          user    comment
-------------------  -----------  ----------------------------  ------  ---------
2020_10_09_04_53_58  warm-reboot  Fri Oct  9 04:51:47 UTC 2020  admin
2020_10_09_02_33_06  reboot       Fri Oct  9 02:29:44 UTC 2020  admin
2020_10_09_02_00_53  fast-reboot  Fri Oct  9 01:58:04 UTC 2020  admin
2020_10_09_01_56_59  reboot       Fri Oct  9 01:53:49 UTC 2020  admin

HLD : sonic-net/SONiC#669

[jleveque]

As comments. Also, since reboot-cause is now a group, can you please move this functionality to a new file, reboot_cause.py and import the file in main.py? There are a number of other examples.

[lgtm-com]

This pull request introduces 1 alert when merging 7182079 into ef5f212 - view on LGTM.com

new alerts:

• 1 for Unused import

[lgtm-com]

This pull request introduces 1 alert when merging 1ac64c6 into ef5f212 - view on LGTM.com

new alerts:

• 1 for Syntax error

[lgtm-com]

This pull request introduces 1 alert when merging 24f8dd0 into 061f428 - view on LGTM.com

new alerts:

• 1 for Unused import

[lguohan]

please paste code coverage number.

[sujinmkang]

retest this please!

[sujinmkang]

retest this please!

[sujinmkang]

retest this please

[sujinmkang]

retest this please

[sujinmkang]

retest this please

[lguohan]

can you fix unit test failure?

[sujinmkang]

@lguohan yes, I'm checking it. I got some other errors on sonic-utilities builds so I'm doing a clean up build to reproduce this unit test failure.

[sujinmkang]

[2020-11-28T07:13:01.701Z] ----------- coverage: platform linux, python 3.7.3-final-0 -----------
[2020-11-28T07:13:01.701Z] Name Stmts Miss Cover
[2020-11-28T07:13:01.701Z] -----------------------------------------------------------------
[2020-11-28T07:13:01.702Z] show/reboot_cause.py 48 4 92%