Proposal: remove background-color from .mat-drawer-container

#### Bug, feature request, or proposal:
Proposal: remove `background-color` from `.mat-drawer-container`

#### What is the expected behavior?
The common background that I've set in my app appears on a page with a drawer

#### What is the current behavior?
Putting a drawer on a page requires putting the page contents inside `md-drawer-container` which comes with `background-color` set that overrides any app background that's been set.

#### What are the steps to reproduce?
Add a drawer to a page with a non-standard background

#### What is the use-case or motivation for changing an existing behavior?
Overlays for drawers and sliders are typically supposed to block the majority of the UI. IMHO material shouldn't be providing a background-color for this majority.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
2.0.0-beta.10

#### Is there anything else we should know?

Inizia cercando `.mat-drawer-container` negli stili di Angular Material e il punto di ingresso del componente `md-drawer-container`, quindi riproduci il problema con uno sfondo dell’app non standard e un drawer. Il lavoro è completato quando lo sfondo dell’app rimane visibile dietro il contenuto della pagina del drawer senza compromettere il comportamento del drawer e i test pertinenti sono stati aggiornati o aggiunti, se quell’area è coperta dai test.

Scritto dal modello di indicizzazione a partire dal testo della issue.