Add gradle to tools installation list

[xbonnamy]

As bootstrapped ML cluster/instance could be used with DHF, it would be nice that gradle would be added to the existing list (Zip/Unzip, Java, MLCP, Nodejs, etc.) of tools to install.

[grtjn]

here are notes on how to do that, but it is not quite a one-liner. I think I'd recommend using ./gradlew over global gradle anyhow.

But if someone would like to take this, and open a PR, I'd welcome it..