bkr: support `--ks-append` for provisioner plugin

[jbpratt]

We often want to just append on a small set of commands onto the existing kickstart file that is provided (if existing). Rather than requiring the entire thing, we can make use of the kickstart append command option on the beaker client.

bkr workflow-simple \
  --whiteboard="example" \
  --family="RedHatEnterpriseLinux8" \
  --tag="RELEASED" \
  --variant="BaseOS" \
  --arch="x86_64" \
  --ks-append="
reqpart
part /boot --recommended
part pv.rhel --grow
volgroup ${vg_name} pv.rhel
logvol /    --vgname=${vg_name} --name=root --size=1 --grow
logvol swap --vgname=${vg_name} --name=swap --recommended
" \
  --task="/distribution/check-install" \
  --machine="${PROVISIONING_HOST}" \
  --debug \
  --prettyxml \
  --wait

[dno-github]

Jira ticket CCITCARBON-347 has been created for this issue. Url : https://projects.engineering.redhat.com/browse/CCITCARBON-347