nvmetcli: Allow different devices for make test

nvmetcli: Allow different devices for make test

The test_nvmet.py by default uses /dev/ram0 and /dev/ram1 for 2 of the
unit tests.  Add env. variable to allow user to specify different devices
or files.  Additionally, skip these unit tests that require devices/files
if they are not present.  Update README too.

$ sudo make test
......s...s.
----------------------------------------------------------------------
Ran 12 tests in 0.043s

OK (skipped=2)
Name                  Stmts   Miss  Cover
-----------------------------------------
nvmet/__init__.py         1      0   100%
nvmet/nvme.py           517    237    54%
nvmet/test_nvmet.py     276     63    77%
-----------------------------------------
TOTAL                   794    300    62%

$ sudo NVMET_TEST_DEVICES="/dev/sdc,/dev/sdd" make test
............
----------------------------------------------------------------------
Ran 12 tests in 0.124s

OK
Name                  Stmts   Miss  Cover
-----------------------------------------
nvmet/__init__.py         1      0   100%
nvmet/nvme.py           517    100    81%
nvmet/test_nvmet.py     276      4    99%
-----------------------------------------
TOTAL                   794    104    87%

Signed-off-by: Tony Asleson <tasleson@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>