[ci]: Purge swss before install (sonic-net#1654)

Observe persistent failure of newly added vs test on testing code change in intfmgrd after migrating to azure pipeline. sonic-net#1521 passed in legacy Jenkins pipeline.

What we found is that the code change in the swss compiled with PR change is not properly installed in vs docker for a direct installation using dpkg -i. This is confirmed by verifying pipeline artifacts that md5sum value of /user/bin/intfmgrd changes if we install swss deb aritfact into the docker vs artifact, while that of /usr/bin/orchangent stays unchanged.

md5sum before swss install in vs docker

# md5sum /usr/bin/orchagent
28307a7805ea6f3bc5057c0257bf46e6  /usr/bin/orchagent
 
# md5sum /usr/bin/intfmgrd  
fa2b06e20be683286adb47c55635a86d  /usr/bin/intfmgrd
md5sum after swss install

# dpkg -i swss_1.0.0_amd64.deb 
(Reading database ... 19180 files and directories currently installed.)
Preparing to unpack swss_1.0.0_amd64.deb ...
Unpacking swss (1.0.0) over (1.0.0) ...
Setting up swss (1.0.0) ...
# md5sum /usr/bin/orchagent
28307a7805ea6f3bc5057c0257bf46e6  /usr/bin/orchagent

# md5sum /usr/bin/intfmgrd 
e959340709e7aedd7489e69dfd19768f  /usr/bin/intfmgrd

Signed-off-by: Wenda Ni <wonda.ni@gmail.com>

.azure-pipelines/docker-sonic-vs/Dockerfile

@@ -13,4 +13,5 @@ RUN dpkg -i /debs/libsairedis_1.0.0_amd64.deb
 RUN dpkg -i /debs/libsaivs_1.0.0_amd64.deb
 RUN dpkg -i /debs/syncd-vs_1.0.0_amd64.deb
 
+RUN dpkg --purge swss
 RUN dpkg -i /debs/swss_1.0.0_amd64.deb