sma: loading out-of-tree plugins

It's now possible to register SMA device managers defined in a module
outside of the regular directory.  To do that, a global variable called
`devices` containing a list of device manager classes needs to be defined
in a module that's added to SMA_PLUGINS environment variable.

For example:
```
$ cat /path/to/plugins/external_devices/__init__.py
import spdk.sma as sma

class MyDeviceManager1(sma.DeviceManager):
	pass

class MyDeviceManager2(sma.DeviceManager):
	pass

devices = [MyDeviceManager1, MyDeviceManager2]

$ SMA_PLUGINS=external_devices scripts/sma.py
```

Multiple plugins can be specified by separating them with a colon, e.g.:
```
SMA_PLUGINS=plugin1:plugin2
```
Of course, the location at which these modules reside must be in
PYTHONPATH.

Signed-off-by: Konrad Sztyber <konrad.sztyber@intel.com>
Change-Id: I2bb4cfea5191710f97d66abe3a21e4deacc6338a
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/11412
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>

scripts/sma.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 
 from argparse import ArgumentParser
+import importlib
 import logging
 import os
 import sys
@@ -34,9 +35,19 @@ def register_device(agent, device):
     agent.register_device(device)
 
 
+def load_plugins(agent, client, plugins):
+    for plugin in plugins:
+        module = importlib.import_module(plugin)
+        for device in getattr(module, 'devices', []):
+            logging.debug(f'Loading external device: {plugin}.{device.__name__}')
+            register_device(agent, device(client))
+
+
 if __name__ == '__main__':
     argv = parse_argv()
     logging.basicConfig(level=os.environ.get('SMA_LOGLEVEL', 'WARNING').upper())
     agent = sma.StorageManagementAgent(argv.address, argv.port)
     register_device(agent, sma.NvmfTcpDeviceManager(get_build_client(argv.socket)))
+    load_plugins(agent, get_build_client(argv.socket),
+                 filter(None, os.environ.get('SMA_PLUGINS', '').split(':')))
     agent.run()