Install tcpdump on Fedora

Tcpdump has a fairly small footprint and should be installed to all
images (it currently is on the ubuntu image by default). Sometimes the
reason you need it is also the reason its to late to get it.

Change-Id: I4849157515b389d534bdee755cdac6b0768fcb29
Implement dhcp-all-interfaces for systemd.

Includes:

 * Enhancements to the generate-interfaces-file.sh script
 so that it generates ifcfg network-scripts for
 distros like Fedora/RHEL.

 * Includes a new dhcp-all-interfaces.service systemd
 script which ensures network interfaces get generated
 before the network service starts on Fedora.

 * Add a new disable_interface function to
 generate-interfaces-file.sh which (on Fedora)
 will delete the ifcfg config for an interface which
 isn't plugged in.
 This is important because ifcfg-eth0 exists by default
 and we want to remove it if the NIC isn't connected.
 Previous behaviour on Ubuntu is unchanged and the
 generate-interfaces-file.sh just logs it as skipped.

 * General doc updates...

I tested this on a multi-nic machine w/ Fedora where the first NIC
exists but was not connected.

Change-Id: Ia99e312539da43caefd72aa60398d43dac5dcc8f
Closes-Bug: 1239880
Merge "Install tcpdump on Fedora"