Update the ML2 configuration in devstack

This commit adds support to devstack to set ML2 options including
MechanismDrivers, TypeDrivers, and TypeDriver specific configuration for ML2
when running with devstack.

This also allows for simple configuration of VLAN, GRE, and VXLAN ML2 devstack
setups with the OVS agent as follows:

    # VLAN configuration
    Q_PLUGIN=ml2
    ENABLE_TENANT_VLANS=True

    # GRE tunnel configuration
    Q_PLUGIN=ml2
    ENABLE_TENANT_TUNNELS=True

    # VXLAN tunnel configuration
    Q_PLUGIN=ml2
    Q_ML2_TENANT_NETWORK_TYPE=vxlan

Fixes bug 1200767

Change-Id: Ib16efac13440b5d50658a0e6be35bc735510a262
Add option to serve PXE for baremetal from nova-network

New variable BM_DNSMASQ_FROM_NOVA_NETWORK: if setting this true,
DevStack configures nova-network's dnsmask to provide PXE and does
not run baremetal's one. In this case PXE traffic occur in the fixed-ip
network, so no dedicated network for PXE is needed.

Change-Id: I67cade02c03ab45ab6b77d8da9066d7d5ec6c78b
Set horizon configuration for security groups

Fix bug #1206271

Change-Id: I33a530265be1e5ab9181e605eb4b7c3bf1fdf9c4
renamed deprecated glanceclient parameter

According to the sources the parameter --public in python-glanceclient
should be removed after only using the new parameter --is-public in
Devstack.

Change-Id: I25fbb23f4823b3766db647dd50a5b538aad3e55a
Merge "Update the ML2 configuration in devstack"
Merge "renamed deprecated glanceclient parameter"
Merge "Set horizon configuration for security groups"
Merge "Add option to serve PXE for baremetal from nova-network"