v2.2.0

FEATURES:

• New Data Source: digitalocean_ssh_keys (#519) Thanks to @stack72!
• New Resource: digitalocean_custom_image (#517) Thanks to @frezbo!

BUG FIXES:

• digitalocean_kubernetes_node_pool: Validate min_nodes is at least 1 and fix example (#525).
• digitalocean_app: Document the database spec (#524).
• digitalocean_container_registry: Update docs w/ subscription_tier_slug (#523).