Enable consistent network device naming

CentOS cloud images come with net.ifnames=0 on the kernel command line,
which disables consistent network device naming. This does not provide a
good experience on bare metal because NIC ordering can vary. This is
specific to cloud images: an ISO installation would use consistent
network device naming.

We now set net.ifnames=1 in the DIB default environment to use
consistent network device naming. The parameters `nofb nomodeset
gfxpayload=text` are also set to preserve defaults from DIB.

To restore the existing behaviour, set DIB_BOOTLOADER_DEFAULT_CMDLINE to
`nofb nomodeset gfxpayload=text net.ifnames=0` in the
kolla_bifrost_dib_env_vars_extra dictionary.

Change-Id: I20465eab4e0aec6620578a92d3bdbddcec0954df