Remove duplicate _check_provider_update from plugins

This patch removes the duplicated method _check_provider_update
and adds a check for it in the NVP plugin that was missing so that an error
is raised if someone tries to update a provider network.

Fixes bug: 1192702

Change-Id: I84e6767f351122f9e5af0cd4ef24d8eaaeb4ccd5
Merge "Remove duplicate _check_provider_update from plugins"