Add str conversion for numeric xcvr info fields to xcvrd (sonic-net#200)

Converted nominal_bit_rate and cable_length fields from the sfp transceiver info dict into str in post_port_sfp_info_to_db.

The documentation for get_transceiver_info in SfpBase indicates that these field values should be integers. However, only string values can be passed into swsscommon.FieldValuePairs. Therefore, a string conversion is needed.

sonic-xcvrd/xcvrd/xcvrd.py

@@ -323,9 +323,9 @@ def post_port_sfp_info_to_db(logical_port_name, table, transceiver_dict,
                      ('ext_identifier', port_info_dict['ext_identifier']),
                      ('ext_rateselect_compliance', port_info_dict['ext_rateselect_compliance']),
                      ('cable_type', port_info_dict['cable_type']),
-                     ('cable_length', port_info_dict['cable_length']),
+                     ('cable_length', str(port_info_dict['cable_length'])),
                      ('specification_compliance', port_info_dict['specification_compliance']),
-                     ('nominal_bit_rate', port_info_dict['nominal_bit_rate']),
+                     ('nominal_bit_rate', str(port_info_dict['nominal_bit_rate'])),
                      ('application_advertisement', port_info_dict['application_advertisement']
                       if 'application_advertisement' in port_info_dict else 'N/A'),
                      ('is_replaceable', str(is_replaceable)),