Cannot find database driver: org.postgresql.Driver

Edit:

The problem was resolved by replacing
driver: org.postgresql.Driver with driver=org.postgresql.Driver in the liquibase.properties file.

---

Original Answer:

You have added the postgresql driver as a dependency of your webapp. But when maven plugins run, they have their own classpath, which is different to your webapp. So you need to include a dependency on the JDBC driver for the plugin itself (same applies to other plugins, e.g. jetty-maven-plugin):

<plugin>
    <groupId>org.liquibase</groupId>
    <artifactId>liquibase-maven-plugin</artifactId>
    <version>2.0.5</version>
    <configuration>
        <propertyFileWillOverride>true</propertyFileWillOverride>
        <propertyFile>src/main/resources/liquibase.properties</propertyFile>
        <changeLogFile>src/main/resources/changelogs/changelog.xml</changeLogFile>
    </configuration>
    <executions>
        <execution>
            <!--  Another Error: plugin execution not covered by lifecycle configuration..-->
            <!-- <phase>process-resources</phase> <goals> <goal>update</goal> </goals> -->
        </execution>
    </executions>
    <dependencies>
        <dependency>
            <groupId>postgresql</groupId>
            <artifactId>postgresql</artifactId>
            <version>9.1-901-1.jdbc4</version>
        </dependency>
    </dependencies>
</plugin>

Following plugin configuration worked for me.

         <plugin>
            <groupId>org.liquibase</groupId>
            <artifactId>liquibase-maven-plugin</artifactId>
            <version>3.5.0</version>
            <configuration>
            <propertyFile>src/main/resources/liquibase.properties</propertyFile>
            </configuration>
            <executions>
                <execution>
                    <goals>
                        <goal>update</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>