python API: work towards python/vpp api separation

python API: work towards python/vpp api separation

This change improves vpp_papi behaviour by introducing alternate way of
calling vpp APIs.

The common code is the same:

vpp = VPP(...)
vpp.connect(...)

Calling VPP API is different, instead of deprecated:

vpp.show_version() # deprecated

one should write

vpp.api.show_version()

this allows VPP messages like "connect" and "disconnect" to be used,
once the old API is dropped (in 17.07). Also part of this patch is a
check for name conflict, to prevent VPP object overwriting its own
functionality with generated code based on json files.

Change-Id: I22e573b6a45f8b2a1f0340c5c2597c194fe42ca4
Signed-off-by: Klement Sekera <ksekera@cisco.com>