How do I skip a Maven plugin execution if "-DskipTests" or "-Dmaven.test.skip=true" is specified?

This worked for me:

<configuration>
   <skip>${skipTests}</skip>
</configuration>

OR

<configuration>
   <skip>${maven.test.skip}</skip>
</configuration>

You could use profiles that get activated when using one of the unit test skip properties to set a new property (e.g. skipLiquibaseRun) that holds the flag if liquibase should run or not

<profiles>
    <profile>
      <id>default</id>
      <activation>
        <activeByDefault>true</activeByDefault>
      </activation>
      <properties>
        <skipLiquibaseRun>false</skipLiquibaseRun>
      </properties>
    </profile>
    <profile>
      <id>skipTestCompileAndRun</id>
      <activation>
        <property>
          <name>maven.test.skip</name>
          <value>true</value>
        </property>
      </activation>
      <properties>
        <skipLiquibaseRun>true</skipLiquibaseRun>
      </properties>
    </profile>
    <profile>
      <id>skipTestRun</id>
      <activation>
        <property>
          <name>skipTests</name>
        </property>
      </activation>
      <properties>
        <skipLiquibaseRun>true</skipLiquibaseRun>
      </properties>
    </profile>
</profiles>

Use the new property in the liquibase plugin section to decide if the run should be skipped, like this:

<configuration>
    <skip>${skipLiquibaseRun}</skip>
    <driver>com.mysql.jdbc.Driver</driver>
    <url>jdbc:mysql://${test.mysql.db.host}:${test.mysql.db.port}/${test.mysql.db.sid}</url>
    <username>${test.mysql.db.user}</username>
    <password>${test.mysql.db.password}</password>
    <changeLogFile>${project.build.directory}/db.changelog-master.xml</changeLogFile>
    <promptOnNonLocalDatabase>false</promptOnNonLocalDatabase>
</configuration>

Not tested, but hope it works ;-)

I think the most straight-forward way is to cascade the "skip" properties in your POM:

<properties>
  <maven.test.skip>false</maven.test.skip>
  <skipTests>${maven.test.skip}</skipTests>
  <skipITs>${skipTests}</skipITs>
</properties>

Then you can use the last "skip" property set above in your plugin's configuration:

<configuration>
  <skip>${skipITs}</skip>
</configuration>

See Maven Failsafe Plugin: Skipping Tests for more details on each of the different "skip" properties mentioned in this answer.