Add Flexible Topologies tables and modify deliveryservice table

[zrhoffman]

I'm submitting a ...

• new feature / enhancement request

Description

Traffic Control components affected ...

• Traffic Ops DB

Acceptance Criteria

• topology table adheres to this schema:

  column	type	modifiers
  name	text	not null, PK
  description	text
  last_updated	timestamptz	default now()

• topology_cachegroup table adheres to this schema:

  column	type	modifiers
  id	int	not null, PK
  topology	text	not null, FK: references topology(name)
  cachegroup	text	not null, FK: references cachegroup(short_name)
  last_updated	timestamptz	default now()

• topology_cachegroup_parents table adheres to this schema:

  column	type	modifiers
  child	int	not null, FK: references topology_cachegroup(id)
  parent	int	not null, FK: references topology_cachegroup(id)
  rank	int	not null
  last_updated	timestamptz	default now()

• After running migrations, deliveryservice table has a topology column that can be described as:

  column	type	modifiers
  topology	text	FK: references topology(name)

The line

See the Flexible Topologies PR for specifics: #4537