1. commands added(see below) and testcases for new commands
2. db_migrator:
   - migrate CONFIG_DB from old approach to the new approach
   - when system warm starts from old image to the new one,
     copies related tables from CONFIG_DB to APPL_DB for the purpose
     that buffermgrd can start smoothly
3. warm-reboot script: don't clear BUFFER_MAX_PARAM table across warm reboot

CLI list
- config interface buffer priority-group lossless <add|set|remove>
  - config interface buffer priority-group lossless add <port> <PG> [headroom-override-profile]
    for adding the PG for the first time
    providing option headroom-override-profile means to configure the PG as headroom override
    otherwise as dynamically calculated headroom
  - config interface buffer priority-group lossless set <port> <PG> [headroom-override-profile]
    for modifying an existing PG, the option headroom-override-profile has the same meaning as "add"
  - config interface buffer priority-group lossless remove <port> [PG]
    for removing the PG specified by option PG.
    if the option isn't provided, all lossless PGs on the port will be removed
- config buffer-profile <add|set|remove>
  To add, modify or remove buffer profiles
- show buffer <configuration|information>

Testcase covered for global config commands.
show command unconvered due to subprocess call isn't supported by test infra

Signed-off-by: Stephen Sun <stephens@nvidia.com>

1. If all the buffer configuration aligns the default, use dynamic
buffer calculation mode. Otherwise, use the traditional mode
2. Dynamic mode is adopted in switches newly installed from scratch
and traditional mode in switches installed from minigraph
This is done by:
- introducing the option --no-dynamic-buffer in "config qos reload"
  to designate whether the dynamic or traditional mode is used
- introducing a new filed "buffer_model" in DEVICE_METADATA|localhost
  to store which buffer model currently is used
- updating db_migrator accordingly

Signed-off-by: Stephen Sun <stephens@nvidia.com>

Review comments:
- Add testcases for show and config command
- Address review comments in db_migrator
- Use "size" to represent the size of the PG and "headroom" for xoff
- Fix typo

Signed-off-by: Stephen Sun <stephens@nvidia.com>

Signed-off-by: Stephen Sun <stephens@nvidia.com>

- Fix alignment error
- Update DEVICE_METADATA|localhost.buffer_model only if it is changed

Signed-off-by: Stephen Sun <stephens@nvidia.com>