add 'install' to features on hammer.py build

Addressing the failures in tarball-system-tests:

 |forge-client-v6 09:19:45.833837| [172.28.0.31] run: /usr/local/sbin/kea-admin db-init mysql -u keauser -p keapass -n keadb
 |forge-client-v6 09:19:45.836172| [172.28.0.31] out: /bin/bash: /usr/local/sbin/kea-admin: No such file or directory

The probable cause is that Kea is not getting installed due to the default features not being automatically taken into consideration in hammer since recent changes.

This MR explicitly adds install to the hammer build command.