Changes way metrics are collected, removing the use of mon-proxy

Collector now collects vim metrics directly, without to send kafka msgs
to mon-proxy. Also a plugin/backend model has been implemented to ease
possible inclusions of other vims and tsbds in the future.

Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>
Change-Id: I554a4f5e410a31ec70aa301c8aa819b1f03a3857