(#401) Checking network status with nm-online

This change checks the network status at the top of the os_boot.erb
template for Red Hat 6 so that the script can connect to the Razor
daemon over the network. This is done by checking the status of
the NetworkManager service and calling the nm-online command, when
it's available, with a reasonable timeout that returns as soon as the
network is online. The command and the timeout values are taken from the
NetworkManager init script from CentOS, so it should be sane and should
not introduce any regressions.

The change is necessary in the context where a Red Hat derivative uses
Anaconda which configures the network with Network Manager. This is an
asynchronous process so the network might still be offline when rc.local
is called so the boot process never completes the handoff to the broker.
Merge pull request #402 from cr3/401

(#401) Checking network status with nm-online