cerbero/commands: FetchBootstrap: add -y/--assume-yes

I would like to use cerbero in an Ubuntu-based CI/CD environment, allowing Cerbero to install whatever packages it requires without user interaction followed by building the various packages (i.e., fetch and then build as separate image layers in a Dockerfile).

Based on the gitlab-ci.yml, I found ci/docker_setup.sh and expected therefore fetch-bootstrap to not require user interaction. However, it does on Ubuntu since the bootstrap.py has the assume_yes=FALSE hard-coded (vs. the Bootstrap class having an arg for choosing TRUE). This patch adds that argument to the FetchBootstrap class.