update vagrant documentation
only check for vagrant-omnibus and vagrant-berkshelf if explicitly using base_image vagrant machine
Tweak to work with the latest vagrant-berkshelf

Tested with vagrant-berkshelf 4.0.0.

Change-Id: I0507a11f0d189695fdf653568feba4e1e6884af2
Whitespace cleanup

Change-Id: Ibc028689e72e393c6f21bf31cc4a40a88165e417