Restrucutre gate scripts

So we can respect DRY and share as much code as possible I have broken
out the common code between the aio and multinode gate scripts.

Additionally, this lays the ground work for removing our policy on
root-everywhere by using sudo. Once we get the non-root stuff worked
out we can gate as non-root user.

Change-Id: I781c597ab10f2296b95f51ae27e0fa617ffe0a66
Partially-Implements: blueprint multinode-gate