This adds support for disabling writeback cache by adding a new
DISABLE_WRITEBACK_CACHE option to "scylla-server" sysconfig file, which
makes the "scylla_prepare" script (that is run before Scylla starts up)
call perftune.py with appropriate parameters. Also add a
"--disable-writeback-cache" option to "scylla_sysconfig_setup", which
can be called by scylla-machine image scripts, for example.
Refs: #7341
Tests: dtest (next-gating)
Closes#8526
0ddbed2513