inserting webhook key in separate table
Deleting from webhook keys table when deleting single webhook
Delete webhook keys when deleting group
webhook_keys table removal when deleting group

Also made deletion query global since it was used in other places
Returning queries and params instead of mutating - per @radix
removed unwanted comment - thanks @cyli
clearer comments - for @radix :)
reordering query args based on schema - thanks @cyli
Merge pull request #698 from rackerlabs/as-222-webhook-table

AS-222 webhook keys table insert and delete