DB hostname comment for Ubuntu based installation

If the DB hostname in the datacube_integration.conf file is set to localhost, the integration tests fail with the message that implies database connectivity issue. This is because for Ubuntu, postgres uses /var/run/postgresql folder. 
The fix was mentioned in this issue: opendatacube#1329

docs/installation/setup/common_install.rst

@@ -92,6 +92,7 @@ Then edit the ``~/.datacube_integration.conf`` with a text editor and add the fo
     db_username: <foo>
     db_password: <foobar>
 
+Note: For Ubuntu Setup the db_hostname should be set to "/var/run/postgresql". For more refer: https://github.com/opendatacube/datacube-core/issues/1329
 
 Verify it all works
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