Fix bug that causes InternalServerError in FT

This patch fixes a bug that caused many functinal-tests
to fail with the following error:
- tackerclient.common.exceptions.InternalServerError

The bug occurs because the password is converted to bytes
type when decrypting the password of vim_auth with python3.

Change-Id: I021d1752fcf96dfba97faf21e849dc87019e4e57
Closes-Bug: #1883350
(cherry picked from commit bdb2d52b3a)

tacker/nfvo/nfvo_plugin.py

@@ -572,7 +572,7 @@ class NfvoPlugin(nfvo_db_plugin.NfvoPluginDb, vnffg_db.VnffgPluginDbMixin,
         if not f:
             LOG.warning('Unable to decode VIM auth')
             raise nfvo.VimNotFoundException(vim_id=vim_id)
-        return f.decrypt(cred)
+        return f.decrypt(cred).decode('utf-8')
 
     @staticmethod
     def _find_vim_key(vim_id):

tacker/vnfm/vim_client.py

@@ -123,7 +123,7 @@ class VimClient(object):
         if not f:
             LOG.warning('Unable to decode VIM auth')
             raise nfvo.VimNotFoundException(vim_id=vim_id)
-        return f.decrypt(cred)
+        return f.decrypt(cred).decode('utf-8')
 
     @staticmethod
     def _find_vim_key(vim_id):