Simplify NVP plugin configuration

Fixes bug #1121605

This patch replacest the nvp_controller_connection configuration option
with a set of options more intutitive to user which also avoid
repetitions in ini files.

In order to simplify the configuration of the plugin, this patch also
removes named clusters. As the plugin supports a single cluster at
the moment, this can be simply specified in the [DEFAULT] configuration
section.

Also, this patch restrucures nvp_cluster.NvpCluster so that per-cluster
configuration options are not store anymore multiple times.

Change-Id: Id5f84220122d7c5f3239e3333cb772247d1ed05e