Refactor migrations and background workers startup.

* Thinner Rake file
* spec_helper config now has :db component

[#60026682]

Signed-off-by: Phan Le <ple@pivotallabs.com>
Move db_encryption_key to the Encryptor

[#60026682]

Signed-off-by: Phan Le <ple@pivotallabs.com>
Rename configure in Config

Signed-off-by: Phan Le <ple@pivotallabs.com>
Rename MessageBus{Configurer,}::Configurer

Signed-off-by: Phan Le <ple@pivotallabs.com>
Rename methods in cloud_controller/runner to clear things up.

[finished #60026682]

Signed-off-by: Phan Le <ple@pivotallabs.com>