install-server: fix late_command for trusty

Remove wikimedia-base from the packages that late_command (&
standard-pacakges) installs. While it's easy to copy it from precise,
it's also a useless package as the only thing that it actually does is
to depend on openssh-server, which we install anyway.

Make the ixgbe-dkms package installation conditional on the version of
the module itself, as our ixgbe patch is not needed in recent kernels
(incl. trusty). While this did no fail on the test system, it would fail
on systems that actually have ixgbe, since we don't (nor plan to have)
ixgbe-dkms on trusty.

Change-Id: I47d40e31fb715972ce045382d290317a9c602361