Bump image_suggestions executors from 9GB to 16GB.
In this MR we bump executor memory from 9 to 16GB for the Spark jobs on image_suggestions.
The job now requires more memory for the executor nodes to be reliable. This is due to the coalesce introduced in https://phabricator.wikimedia.org/T323614 creating bigger parquet files, which require more memory for buffering before writing.
We also make sure this setting is now a varprop for easier tinkering in future.