Consolidate rootwrap settings for neutron element

Moves the root_helper settings for the DHCP and OVS agents
into the common neutron.conf file.

This actually removes a deprecation WARNING from the dhcp agent
logs as well:

Deprecated: DEFAULT.root_helper is deprecated! Please move root_helper
configuration to [AGENT] section.

Change-Id: I79251984ab86b34590dc89139f3c7d02d6175714
Merge "Consolidate rootwrap settings for neutron element"