Don't install openvswitch-datapath-dkms on newer kernels

On kernels >= 3.13 for Ubuntu, there is no need to install the
openvswitch-datapath-dkms package anymore. Consequently we don't need the
dkms package anymore, nor the linux headers.

Update the opendaylight devstack code to correctly check for this and make
the right decision.

While here, also utilize get_packages() and common code where possible.

Change-Id: Idd6a71951f6f77b6e3c4e536e735dfead6a40bc1
Closes-Bug: #1331111
Merge "Don't install openvswitch-datapath-dkms on newer kernels"
Merge branch 'master' of github.com:openstack-dev/devstack