Adds python IP utility library

Adds python IP utility library

Changes include:
 - IP utility library in python 3 that supports both IPv4 and IPv6
   address generation. This library currently includes a single
   function of generating IP ranges or single IP for a given CIDR.
   More functionality will be added at a later time to support
   features such as IP address calculation.
 - Updated common-function.sh to use python library to generate IP
   ranges. All existing bash functions are preserved, so any
   callers will get identical IP ranges as before.
 - Add dependency to python3 for opnfv-apex-common package.
 - Add python dependency to build.sh

No change is made to interface related functions.

Change-Id: Idc6998754f9f3c7a3868ec5b5768f3bb5f78cd90
Signed-off-by: Feng Pan <fpan@redhat.com>