Support YCSB_TAR_URL override

It might be better to be able to override `YCSB_TAR_URL`.
Currently, we can only install the complete ycsb-0.7.0 jar, which is about 370MB.
Databses like Cloud Datastore is only well supported after 0.9.0.

Support for overriding ycsb version makes adding new benchmarks easier. Besides, for existing ycsb benchmarks like `cloud_bigtable_ycsb`, `mongodb_ycsb`, `hbase_ycsb`, we can specify the specific ycsb-bindings tar URL to install. Each ycsb-binding shares the same folder structure and the size(about 10MB) is much smaller than the complete ycsb.

There are many benefits to add this feature.