Make auto service optional

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