feat(network-legacy): send dhcp in parallel on all devices

feat(network-legacy): send dhcp in parallel on all devices

We can drastically improve boot times on some machines, or BM or VM
shapes, if we send DHCP requests in parallel on all interfaces
Add ip=single-dhcp as a kernel boot parameter, to send DHCP requests in
parallel on all interfaces. Thus, DHCP on each interface is overlapped
thereby reducing boot times for multiple interface machines.
This can help reduce the boot time by 1 minute per interface.
In addition, we may also save another 40 secs delay per interface, if
the link on which we try to send DHCP is in DOWN state. This can be
reduced by giving the following kernel command line parameter while
booting - rd.net.timeout.carrier=<timeout>.
Added manpage for ip=single-dhcp with the caveat that it does not
apply to Network Manager or SUSE wicked.

Signed-off-by: Anjali Kulkarni <anjali.k.kulkarni@oracle.com>