add support for Horizon's caches backend configuration (memcached)

With this we add into the Horizon element support for two new
keys meant to configure the Horizon's session storage mechanism with
memcached.

Related to blueprint tripleo-icehouse-ha-production-configuration

Change-Id: Ife7137054eafba548d13c58265abd6373679794e
openvswitch and openvswitch-datapath element

Split out the openvswitch installation from the neutron element.

Don't install openvswitch dkms modules in kernel >= 3.12
Openvswitch GRE and VXLAN capabilities are now included in
vanilla kernels (GRE tunneling support in 3.11 and VXLAN in
3.12) so there is no need to install the dkms packages to activate
them. When installing the datapath package, dkms will fail at
compile time with an unsupported kernel version message.

Change-Id: Id456cfb1a0634f43617c23ac82fc4445a58ddb2c
Partial-bug: #1316985
Merge "add support for Horizon's caches backend configuration (memcached)"