This commit adds the ability to specify a plugin from maven for a
test cluster to use. Currently, only local projects may be used as
plugins, except when testing bwc, where the coordinates of the project
are used. However, that assumes all projects always keep the same
coordinates, or are even still plugins, which is no longer the case for
x-pack. The full cluster and rolling restart tests are changed to use
this new method when pulling x-pack versions before 6.3.0.
be8c094e8c