PAPI: Allow ipaddress object as argument and return values from API calls

The API calls that use any of vl_api_address_t, vl_api_ip4_address,
vl_api_ip6_address_t, vl_api_prefix_t, vl_api_ip4_prefix_t,
vl_api_ip6_prefix_t now accepts either the old style dictionary,
a text string (2001:db8::/32) or an ipaddress ojbect.

Unless it is called with '_no_type_conversion':True, it will
also return an appropriate ipaddress object.

Change-Id: I84e4a1577bd57f6b5ae725f316a523988b6a955b
Signed-off-by: Ole Troan <ot@cisco.com>