[docker-sonic-mgmt]: Add the snmp tool to the sonic-mgmt docker (#4110)

The snmp tool is required for interacting with certain type
of PDU hosts in platform PSU/power related testing. This
change is to have the snmp tool pre-built in the sonic-mgmt
docker image.

Signed-off-by: Xin Wang <xinw@mellanox.com>

dockers/docker-sonic-mgmt/Dockerfile.j2

@@ -23,6 +23,7 @@ RUN apt-get install -y \
                 curl \
                 cmake \
                 tcpdump \
+                snmp \
                 python-dev \
                 python-scapy