For modification Tomcat context params, add to context config file (and modify according to your credentials):

<Parameter name="jdbc.driverClassName" value="org.postgresql.Driver" override="false" />
<Parameter name="jdbc.url" value="jdbc:postgresql://localhost/clover?charSet=UTF-8" override="false" />
<Parameter name="jdbc.username" value="postgres" override="false" />
<Parameter name="jdbc.password" value="" override="false" />
<Parameter name="jdbc.dialect" value="org.hibernate.dialect.PostgreSQLDialect" override="false" />

Or If you use properties file for configuration:

jdbc.driverClassName=com.microsoft.sqlserver.jdbc.SQLServerDriver
jdbc.url=jdbc:postgresql://localhost/clover?charSet=UTF-8
jdbc.username=postgres
jdbc.password=
jdbc.dialect=org.hibernate.dialect.PostgreSQLDialect

Do not forget to add jar with JDBC driver to the Tomcat classpath.