Adjust PVC size defaults for cloud provider constraints

[michaelstingl]

Problem

During testing of #125 on Rackspace, discovered that current default PVC sizes can exceed cloud provider limits:

• Default data PVC: 30GB
• Some providers (e.g., Rackspace) limit SSD volumes to 5-20GB
• Causes deployment failures with error: 'size' parameter must be between 5 and 20

Current Workaround

Users must explicitly override sizes in their values:

opencloud:
  persistence:
    data:
      size: 15Gi  # Must be explicitly set

Proposed Solutions

Option 1: Lower defaults

Set more conservative defaults that work everywhere:

persistence:
  data:
    size: 15Gi  # Instead of 30Gi

Option 2: Cloud provider profiles

Add commented examples in values.yaml:

# Cloud provider examples:
# Rackspace (SSD max 20GB):
#   data.size: 15Gi
#   config.size: 5Gi
# AWS/GCP (no strict limits):
#   data.size: 50Gi

Option 3: Document in README

Add a troubleshooting section about provider-specific limits.

Impact

Prevents deployment failures for users on constrained cloud providers while maintaining flexibility for others.