trusty-backports haproxy no longer needed for ipv6 as of liberty

[Edward Hope-Morley]

As of the Openstack Libery release, the Ubuntu Cloud Archive will carry haproxy (starting with 1.5.*) so we will no longer need to install haproxy from trusty-backports i.e. each charm currently has:

    # NOTE(xianghui): Need to install haproxy(1.5.3) from trusty-backports
    # to support ipv6 address, so check is required to make sure not
    # breaking other versions, IPv6 only support for >= Trusty
    if ubuntu_rel == 'trusty':
        add_source('deb http://archive.ubuntu.com/ubuntu trusty-backports'
                   ' main')
        apt_update()
        apt_install('haproxy/trusty-backports', fatal=True)