Add possibility to just pass key name.

From now, it's possible to just pass the key name, not the full path to
the key. By default it will be looked for in ~/.ssh directory.

box.py

@@ -83,6 +83,9 @@ class VMCreate:
 
         if not self.ssh_key_path.endswith('.pub'):
             self.ssh_key_path += '.pub'
+        if not os.path.exists(self.ssh_key_path):
+            self.ssh_key_path = os.path.join(os.path.expanduser("~/.ssh"),
+                                             self.ssh_key_path)
         if not os.path.exists(self.ssh_key_path):
             raise BoxNotFound(f'Cannot find default ssh public key: '
                               f'{self.ssh_key_path}')