NeutronWrapper class and Network context lacks support of IPv6 subnets and multiple networks

Description of problem:
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NeutronWrapper class and Network context lacks support of IPv6 subnets:
a. (IPv6) Missing default IPv6 start CIDR
b. (multiple nets) currently create_network creates it own router, missing the option to connect it to an existing router
c. (multiple nets) missing delete_router: currently delete_network deletes the router created via create_network. netwapper should support router deletion when it's not created via create_network (for routers with multiple networks)
d. (multiple nets) netwapper improvements: missing the options to list ports, list routers, list router ports
e. (IPv6 & multiple nets) network context setup: does not handle the option of additional IPv6 network.
f. (IPv6 & multiple nets) network context cleanup: does not handle the option standalone routers cleanup.

This prevents for scenarios to use the netwrapper both for multiple networks and IPv6 testings.

all of those issues are linked and will be addressed with a patch.