vpp_papi: Add help() support for python interactive mode

vpp_papi: Add help() support for python interactive mode

The python api 1.3 removed the ability to call 'help' in interactive
mode.

example:
> help(vpp.sw_interface_span_enable_disable)
Help on function sw_interface_span_enable_disable in module vpp_papi.vpp_papi:

sw_interface_span_enable_disable(**kwargs)
    u16 _vl_msg_id, u32 client_index, u32 context, u32 sw_if_index_from, u32 sw_if_index_to, u8 state

Change-Id: Iea1944fdc862482490174ff966592cf3c8208a98
Signed-off-by: Christophe Fontaine <christophe.fontaine@qosmos.com>