[syseepromd] Support both new platform API and old platform plugins

[stephenxs]

Support new platform api with plugin mode compatible.
In this PR, new platform api is supported in following steps:

1. initialization, load new-platform-api-based class eeprom or chassis for syseepromd and psud respectively, if failed then load old style plugin.
2. for each api, call new platform api, old style plugin is called if the former raised a NotImplementError; other Exceptions raised will be passed to caller without handling.

now only two APIs are called in syseepromd, update_eeprom_db and read_eeprom.
five three APIs for psud, get_num_psus, get_presence and get_powergood_status.
it depends on the sonic-platform-common #48 add get_eeprom to Chassis for syseepromd to access

[jleveque]

This is exactly what I was picturing; a simple change, as small as possible, which allows us to share as much of the existing logic as possible between the old plugins and new API.

[jleveque]

@stephenxs: Do you mind splitting the syseepromd and psud changes into separate PRs? It will make the changes more modular in case of the need to revert anything.

[stephenxs]

@stephenxs: Do you mind splitting the syseepromd and psud changes into separate PRs? It will make the changes more modular in case of the need to revert anything.

Done.
PR 36 for syseeprom and PR 37 for psud

[jleveque]

So shall we close this PR?

[stephenxs]

sure.