test/autotest: expand PYTHONPATH to include SPDK RPC lib

When a new python script wanted to use RPC client or calls
from RPC lib from ./spdk/scripts/* it had to be created
within that directory.
By expanding PYTHONPATH in autotest_common, now any python
scripts can use that library.

First example of this is located in:
./spdk/test/bdev/bdevio/tests.py

Change-Id: Idf3e5891c3815a84c70525ab9338100acbfa4a43
Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/455219


Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>