Add use of guestagent install shell script in infra dibs

all dibs elements have to be placed in the same directory
otherwise we cannot execute diskimage-builder command with
multiple elements. In our case we have to use guestagent
element with each infra element (drone, jenkins, gitolite).

Change-Id: I6e55350630e982adec1b338309421c7bdca4c9c3
Merge "Add use of guestagent install shell script in infra dibs"