[docker-snmp] Install pyyaml in the container (sonic-net#2533)

This dependency is needed for arista plugins to properly work.

dockers/docker-snmp-sv2/Dockerfile.j2

@@ -35,6 +35,9 @@ RUN dpkg_apt() { [ -f $1 ] && { dpkg -i $1 || apt-get -y install -f; } || return
 RUN curl https://bootstrap.pypa.io/get-pip.py | python3.6
 RUN python3.6 -m pip install --no-cache-dir hiredis
 
+# Install pyyaml dependency for use by some plugins
+RUN python3.6 -m pip install --no-cache-dir pyyaml
+
 {% if docker_snmp_sv2_whls.strip() -%}
 # Copy locally-built Python wheel dependencies
 {%- for whl in docker_snmp_sv2_whls.split(' ') %}