Install libyang to azure pipeline (sonic-net#268)

#### Why I did it
sonic-swss-common lib will add dependency to libyang soon, so need install libyang lib to prevent build and UT break.

#### How I did it
Modify azure pipeline to install libyang in azure pipeline steps.

#### How to verify it
Pass all UT.

#### Which release branch to backport (provide reason below if selected)

#### Description for the changelog
Modify azure pipeline to install libyang in azure pipeline steps.

#### Link to config_db schema for YANG module changes

#### A picture of a cute animal (not mandatory but encouraged)

azure-pipelines.yml

@@ -47,7 +47,7 @@ stages:
     - script: |
         set -ex
         sudo apt-get -y purge libhiredis-dev libnl-3-dev libnl-route-3-dev
-        sudo dpkg -i ../target/debs/buster/{libswsscommon_1.0.0_amd64.deb,python3-swsscommon_1.0.0_amd64.deb,libnl-3-200_*.deb,libnl-genl-3-200_*.deb,libnl-nf-3-200_*.deb,libnl-route-3-200_*.deb,libhiredis0.14_*.deb}
+        sudo dpkg -i ../target/debs/buster/{libyang_1.0.73_amd64.deb,libswsscommon_1.0.0_amd64.deb,python3-swsscommon_1.0.0_amd64.deb,libnl-3-200_*.deb,libnl-genl-3-200_*.deb,libnl-nf-3-200_*.deb,libnl-route-3-200_*.deb,libhiredis0.14_*.deb}
         sudo python3 -m pip install ../target/python-wheels/buster/swsssdk*-py3-*.whl
         sudo python3 -m pip install ../target/python-wheels/buster/sonic_py_common-1.0-py3-none-any.whl
         python3 setup.py bdist_wheel