Deduplicate AWS and GCP cluster bootstrap by extracting shared tasks (#463)

Extract common cluster initialization, defaults, and VIP management
into three shared task files included by both aws-cluster-bootstrap
and gcp-cluster-bootstrap. Provider-specific logic (credentials,
STONITH, cluster IP) remains inlined in each bootstrap wrapper.
This eliminates ~300 lines of duplicated Ansible tasks while keeping
provider-specific steps easy to follow without file indirection.
New shared files:
- cluster-bootstrap-init.yaml: corosync/pacemaker setup, authkeys
- cluster-bootstrap-defaults.yaml: stonith, stickiness, timeouts
- cluster-bootstrap-vip.yaml: cluster IP locate/move/clear

Author: mpagot