Update catalog, plan, service validation errors

[Finishes #62854062]
Add default value for maximum_app_disk

We want to decouple dependency of cloud controller on cf-release. So
there should be a reasonable default value in CC.

[#64174044]

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
Change the error for non-unique service labels to match the desired error.

- the story specifies a desired error to return when trying to add
  services with the same label.
- uses rescue-less method for updating service label
- ensures uniqueness of label field for v2 services

[#63093570]
Reduces possible test pollution from fake servers

- adds at_exit hooks to specs that run fake service brokers
- finds and kills existing fake broker processes on start

[finishes #64709132]
/v2/services can be filtered by label and provider

[#63185416]
Do not export system_env_json with get /v2/app/:guid response [finishes #55332776]
Add 'purge' parameter to DELETE /v2/services/:guid

A delete request with the param purge=true will delete the service and
its service plans, service instances, service bindings and service plan
visibilities. The service broker will not be contacted for service
instance or binding deletes.

Due to our reliance on before_destroy and after_destroy hooks to enforce
business logic, we were not comfortable doing a Sequel::Model#delete on
each record. Instead, we set the "purged" field on the services model to
true, which replaces the service broker client with a null object. This
ensures that we do not send requests to the broker, but still executes
all before_destroy and after_destroy logic.

[#63185416]
Merge remote-tracking branch 'origin/master' into services-fork

Conflicts:
  lib/cloud_controller/config.rb
  spec/config_spec.rb
  spec/fixtures/config/minimal_config.yml
Adds endpoint for bulk updating service instances

- only allows the service_plan_guid as an updatable attribute

[#63080352]
Refactors ServiceInstance controller internals
Merge remote-tracking branch 'origin/master' into services-fork
Add migration to reset service provider default for mysql
Add dashboard_client_id to service model [#58040510]
Add uaa client info to configuration [#58040510]
Brokers can have a uaa client created by requesting it in their
catalog  [#58040510]
Update error messages [#65060492]
Rename dashboard_client_id to sso_client_id

NB: This changes a migration, but it has not been deployed to any environments yet so we are ok to change it.
Add services api spec for docs [#61862446]
Add api doc specs for service_plans [finishes #61862446]
Do not show duplicate error messages

Also changes capitalization of error messages [#62854062]