Avoid using deprecated Q_AGENT_EXTRA_AGENT_OPTS in ML2 setup

Q_AGENT_EXTRA_AGENT_OPTS is deprecated now and stack.sh displays
the warning that it will be removed early in "K" cycle.
Neutron ML2 devstack uses Q_AGENT_EXTRA_AGENT_OPTS to pass tunnel
configurations to Neutron plugin agents.

This commit removes the usage of Q_AGENT_EXTRA_AGENT_OPTS and
configures these config options directly.

Change-Id: I7fc1613ff78c0a8b1e80cc041b06f5d4680c34f2
Closes-Bug: #1354670
Merge "Avoid using deprecated Q_AGENT_EXTRA_AGENT_OPTS in ML2 setup"