Raises BadRequest for grant_access on ignore_dbs

Reasons:
- Currently, it just logs an informative log in guestagent log, and then
  control shifts to next line, which provides the requested grant.
- This brings the need to raise the exception instead of informative
  logging.

Changes:
- Raises BadRequest if ignore_dbs are requested access instead of
  informative logging into guest-agent logs.

Change-Id: I9df08d04cd42916f3d1f91e05cd4799bb9041eae
Closes-Bug: #1257021
Merge "Raises BadRequest for grant_access on ignore_dbs"