alias bash completion and modular seed configuration

[Diaphteiros]

The aliases have been reworked. The following aliases will work within the docker container now:

k => kubectl
ks => kubectl -n kube-system
kg => kubectl -n garden
kn => kubectl -n
ka => kubectl get --all-namespaces

Bash completion works for all aliases except ka.

Furthermore, it is now possible to deploy additional seeds. By default, only the seed belonging to the cloud provider where the Gardener is running on is deployed (same as before). To add other seeds, list them in the seed_config.seeds node in the landscape_config.yaml or deploy them manually later by calling components/deploy.sh seed-config <seed1> <seed2> ....