Added NetworkExplorer engine object class

Added a NetworkExplorer class (maps to io.murano.system.NetworkExplorer) to explore
the Network Environment of an active tenant.
The class is able to locate the default router (if present) and allocate available
CIDR range for the selected router. The latter requires some configuration options
(for proper CIDR segmentation)

This commit adds python-neutronclient to the requirements, as NetworkExplorer has
to directly interact with Neutron

This classes is a crucial part of AdvancedNetworking implementation

Partial-Bug: #1308921

Change-Id: Ib9daa1b1521d9bc17a53d7e131be6c9f43faa013
Merge "Added NetworkExplorer engine object class"