system-test: Add support for compatibility testing releases

Adds support for installing older releases into the system-test build and
allows these to be tested for compatibility with the current build.

The packages/system-test/target/bin/ctlcluster utility operates on the entire
'cluster' of dCache versions, covering both the current build and other
releases that have been installed. Another release is installed using the
'install' subcommand, eg 'ctlcluster install 2.7.5'.  The switch subcommand is
used to move service definitions between versions, eg 'ctlcluster switch admin
2.7.5' moves the admin service to version 2.7.5 and 'ctlcuster switch admin'
moves it back to the current build. Both the service name and a unique property
assignment for the service can be used as identification, eg 'ctlcluster switch
pool.name=pool_write 2.7.5'.

Other releases installed through the 'ctlcluster install' command are
configured to point to the same databases, some of the same configuration (like
certificate stores) and the same plugins (in particular the hsqldb plugin). The
script cannot compensate for differences in property names or schema changes.
This is unlikely to be a problem when testing other releases of the same major
branch, but may be an issue when testing against older feature releases. In
that case the configuration has to be adjusted by hand.

Several other commands that releate to the system-test package have been moved
out of target/dcache/bin and into target/bin: hsqldb, populate, test,
hostCredentials.

The default system-test.conf layout has been adjusted to make the service
definitions easier to move between releases.

The Maven build has been configured to stop the entire dCache cluster before
cleaning the target directory. This is to avoid orphaned dCache instances.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6585/