Add branding support to ent cert importer.

cli "import" and the "Import Certificates" gui
use managerlib.ImportFileExtractor for writing
import entitlement certificates. Add support
for get_cert() to get the EntitlementCertificate
object.

Add calls the entbranding.BrandInstaller to places
that use ImportFileExtractor.
Add BrandPicker and Brand base class.

BrandPicker is responsible for finding any brand
info. Current version does this based on the passed
in ent certs, and the installed products. It could
be extended to check the candlepin server for
info.

Brand base class is to hold the brand compare logic.