core: Merge VdcUser and DbUser

Currently we have three different classes to represent information about
users: VdcUser, DbUser and LdapUser. The VdcUser and DbUser classes are
almost identical, so they can be merged. This change removes the VdcUser
class and changes the engine to use DbUser instead.

Change-Id: I2ed315043858260ba997203b8e3fc05feb0bad20
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>