NEXUS-5602: WL init add significant startup time.

This change now splits "WL init at boot" from "WL init
at repo addition on runtime" cases. Before, both were
doing same, but due to "cascade", one Nexus with lot of
repo and/or groups booted slower than before.

Now the "cascade" is not done in case of "boot" but on
"repo add at runtime" case is still done (and is needed).
On boot, extra step is done to explicitly init group reposes
WL, instead relying on having them updated implicitly due to
cascade.

This also meant, while former solution was correct, it was
sloppy as same group was updated over and over again on boot
process.