Skip osd-devices not absolute paths

This change skips over any devices which does not start with a leading
folder separator ('/'). Allowing such entries causes an OSD to be
created out of the charm directory. This can be caused by something as
innocuous as 2 spaces between devices. The result is that the root
device is also running an OSD, which is undesirable.

Change-Id: I0b5530dc4ec4306a9efedb090e583fb4e2089749
Closes-Bug: 1652175

actions/add_disk.py

@@ -59,9 +59,13 @@ def add_device(request, device_path, bucket=None):
 
 
 def get_devices():
-    return [
-        os.path.realpath(path)
-        for path in action_get('osd-devices').split(' ')]
+    devices = []
+    for path in action_get('osd-devices').split(' '):
+        path = path.strip()
+        if os.path.isabs(path):
+            devices.append(path)
+
+    return devices
 
 
 if __name__ == "__main__":