test/rocksdb: only use 2GB cache for per-patch testing

We can still use 4GB for nightly testing, but cutting
the cache size to 2GB reduces the setup time for each
of the 5 db_bench test runs.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: Ic946cfcd2ecac209e0e06b05777c18ccdec50d14

Reviewed-on: https://review.gerrithub.io/436564


Chandler-Test-Pool: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>