Move service_down_time to nova.service

blueprint: scope-config-opts
Change-Id: I5fddb3768348c43a38b72dbf738b0c7fa2967691
Move memcached_servers opt into common.memorycache

Add a factory function to nova.common.memorycache which consolidates
the code to choose between real or fake memcache. This then means that
memcached_servers is used in just one place and we can move the config
option there.

blueprint: scope-config-opts
Change-Id: I67c191e0db58364eda4162b9e881606063509b9d
Move global service networking opts to new module

The my_ip, host and use_ipv6 options are used all over the codebase
and they're pretty well related to each other. Create a new netconf
module for them to live in.

There are now no options registered globally in nova.config!

blueprint: scope-config-opts
Change-Id: Ifde37839ae6f38e6bf99dff1e80b8e25fd68ed25
Merge "Move service_down_time to nova.service"
Merge "Move memcached_servers opt into common.memorycache"
Merge "Move global service networking opts to new module"