Make auto service optional

This makes dependencies consistent and eliminates warnings of the
form:

$M2_HOME/repository/org/apache/jclouds/driver/jclouds-slf4j/2.1.0-SNAPSHOT/jclouds-slf4j-2.1.0-SNAPSHOT.jar(org/jclouds/logging/slf4j/config/SLF4JLoggingModule.class): warning: Cannot find annotation method 'value()' in type 'AutoService': class file for com.google.auto.service.AutoService not found

Reference:

https://github.com/google/auto/tree/master/service#download

apis/openstack-neutron/pom.xml

@@ -110,6 +110,7 @@
       <groupId>com.google.auto.service</groupId>
       <artifactId>auto-service</artifactId>
       <scope>provided</scope>
+      <optional>true</optional>
     </dependency>
     <dependency>
       <groupId>com.google.auto.value</groupId>