nfvbenchvm: fix broken build after dib update

nfvbenchvm: fix broken build after dib update

Attempting to build nfvbench VM images with diskimage_builder (dib)
3.16.0 leads to the following error:

diskimage_builder.element_dependencies.MissingElementException: Element
'block-device' not found

Something changed at some point of diskimage_builder history, and the
updated doc states: "When using the vm element, an element that provides
block-device should be included".

To fix this and prevent future similar issues, this patch:
- adds the "block-device-mbr" element to the dependency list
- sets diskimage_builder version

By the way:
- replace the deprecated "centos7" element with the more generic
  "centos" element.
- add missing dependencies in the pre-requisites section of nfvbenchvm
  doc
- install pip with package manager (the current latest version of
  get-pip.py is not compatible with python 3.6, so let's use the
  occasion to stop getting pip directly from the Internet)
- use Python 3 in build-image.sh

Change-Id: I3198a1042eca04224b2a62db443c39a76903cf22
Signed-off-by: Gwenael Lambrouin <gwenael.lambrouin@orange.com>