topo: Add host count param to LinearTopo

Previously, LinearTopo took one parameter (k), which controlled the number
of switches; each of these got one host.  This adds a second parameter (j),
which controls the number of hosts per switch, defaulting to 1 (as before).

This is the second version of this patch, which attempts to make the
host name generation more straightforward.
Merging MurphyMc LinearTopo pull request
Merge branch 'master' of github.com:mininet/mininet