Obtain management port speed from system file

[vincentchiang-ec]

Obtain the actual management port speed from system file /sys/class/net/eth0/speed.

- What I did
Currently the management port speed was obtained from the "speed" value in "MGMT_PORT" table of config_db. However, this field has no actual function.
This PR is to obtain the actual management port speed from system file "/sys/class/net/eth0/speed".

- How I did it
Add a function to obtain the actual management port speed from system file "/sys/class/net/eth0/speed".

- How to verify it
Run snmpwalk to get if high speed, make sure the management port speed is correct.

docker exec -it snmp snmpwalk -v2c -c public 127.0.0.1 1.3.6.1.2.1.31.1.1.1.15
iso.3.6.1.2.1.31.1.1.1.15.1 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.5 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.9 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.13 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.17 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.21 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.25 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.29 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.33 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.37 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.41 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.45 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.49 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.53 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.57 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.61 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.65 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.69 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.73 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.77 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.81 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.85 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.89 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.93 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.97 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.101 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.105 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.109 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.113 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.117 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.121 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.125 = Gauge32: 100000
iso.3.6.1.2.1.31.1.1.1.15.10000 = Gauge32: 1000

- Description for the changelog