Remove remaining uses of passing root_helper

Remove remaining uses of passing root_helper

This should remove the last bits of passing root_helper around
for execute(). Some tests store and pass the root_helper argument
outside of this, and have been left alone.

Depends-On: I1e3b64e5a1d6cff2aebc638710487bbdbdba61d4
Depends-On: I5d51fa5edc7f427d84d7ae49d3a4c972dca8ac37
Depends-On: I21c6e7795c8b2e8f87bbd2b00a8eb7cb215ece74
Depends-On: I6566e17a1ce07a3bebf2a1a3653ad7e7f397f0b9
Change-Id: I38c901c65fb0c8598589ec56d5afd50d2f882f4f
Partially-Implements: blueprint rootwrap-daemon-mode