system-test: Enabled tiered compilation

This change improves startup speed for the dCacheDomain in system-test setups.
Although dCache in general is not sensitive to startup speed, system-test is
restarted many times during development and startup has an effect on developer
productivity.

Since the process isn't necessarily short lived like our client tools, I did
not limit the compilation level.

On my laptop this cut about 5 seconds from process start to the last pool being
enabled.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6694/