autotest_common.sh: increase read timeout inside rpc_cmd

Currently we use 5 seconds as a timeout inside rpc_cmd's
"while read" loop, which is enough for a system with
a single NVMe. That value is insufficient for machines
with more devices and causes the timeout, erroring out
the tests (verified with the same number of devices,
as in the CI node with the most NVMes available - 4).

For that reason increase the read timeout to 15 seconds.

Fixes #2641

Change-Id: If7e9fd169a75ea16ec243c63821a4e5d70995063
Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/14525


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>