Switch VPP package repository to packagecloud.io

VPP install guide [1] now refers to packagecloud.io instead of
nexus.fd.io as the source for VPP binary packages.

[1] https://fd.io/docs/vpp/v2106/gettingstarted/installing/centos.html

I could not find when the switch took place, but I recently got some
intermittent issues when trying to use nexus.fd.io (Bad Gateway errors),
so it seemed like a good opportunity to make the move.

Additionally, this will make it possible to get vpp releases higher
than 19.08 when building nfvbench loop VM image.

Signed-off-by: Gwenael Lambrouin <gwenael.lambrouin@orange.com>
Change-Id: I9cff7289e426e0ed8c8a7df8322a4a4baeb79f47