[Mellanox]Fix issue #2720 Not able to read out values of voltage/temp/power on some cables

[stephenxs]

- What I did
Fix issue #2720 Not able to read out values of voltage/temp/power on some cables, which is due to dom reading error on Mellanox switches. Fix it by reading dom via ethtool rather than sysfs and limit all the modifications within Mellanox-specific code.

- How I did it
A new class based on SfpUtilBase and a new method _read_eeprom_specific_bytes_via_ethtool have been introduced in order to change the way the eeprom DOM data is read. Typically the best practice to do this kind of thing is to limit the modification within the function which executes reading operations only and keep other stuff (especially the interface) untouched. However, this can hardly be achieved since the original reading function takes the file object as an input parameter to represent the port. It is done by having the file object pointed to /var/run/hwmanagement files, which will not be maintained in the future.
As a result, the following reading interface has to be introduced with an explicit port number/name as an input parameter in order to get rid of the dependency on those files:
_read_eeprom_specific_bytes_via_ethtool
Since the interface changed, all methods that call the interface should also be overwritten in order to call the new interface, including:
_read_eeprom_devid
get_transceiver_info_dict
get_transceiver_dom_info_dict

Only interface used to read eeprom DOM has been replaced and the main logic has not been changed except the following mentioned:

1. reading DOM data for sfp port, which is implementioned in get_transceiver_dom_info_dict. In this case a "calibration" should be firstly read from eeprom before other values like temperature, voltage, rx/tx power, can be parsed. However, this has been ignored in the original code, resulting in that the data can't be parsed.
2. In the original implementation, the data area containing the data is read from DOM separately in order to avoid read unnecessary data and achieve better performance. Having used ethtool to read DOM data, the performance gap between reading all the area and reading the spot data separately has been narrowed to almost zero. To make the code neat and readable, we change the way of reading this data to reading the DOM data in a batch.
3. In function get_transceiver_dom_info_dict, a default dict is initialized with all data set to 'N/A' and is returned when ethtool fails to read any dom data. This is because the ethtool returns None for ports without dom support, which results in None being returned by get_transceiver_dom_info_dict and thus failing xcvrd to add the TRANSCEIVER_DOM_SENSOR table entry of those ports to STATE_DB.

- How to verify it
1.execute the following command on both SFP and QSFP ports
sudo sfputil show eeprom -d -p Ethernetxxx
sudo sfputil show eeprom -p Ethernetxxx
2.make sure that TRANSCEIVER_INFO and TRANSCEIVER_DOM_SENSOR table in STADE_DB exist for all ports and contain correct data.

- Description for the changelog
Fix issue #2720 Not able to read out values of voltage/temp/power on some cables, which is due to dom reading error.

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