Fix for issue #172

Per Issue #172 

This commit adds the removal and re-creation of the serval mdp key to the loadPlugin creation process. This will force a key to be created when a user clicks the add button.

To Test: 
1) Flash a node with this branch included
2) Complete the setup wizard
3) Go to the “Basic –> Security –> Shared Mesh Keychain” menu
4) SSH into the device and type the following on the command line
```
SERVALINSTANCE_PATH=/etc/commotion/keys.d/mdp serval-client keyring list
```
5) Keep the value that is given to you. That is the current (default) keyring.
6) In the menu Click the add button
7) type the same command in again
```
SERVALINSTANCE_PATH=/etc/commotion/keys.d/mdp serval-client keyring list
```
8) The output should be different at this point. This means that it is a new keyring.
9) Click the "Save" button
10) The value for "olsrd.cfg144e54.sid=..." should match the latest key.  If it does, then you should merge this pull request as I have fulfilled my aim of fixing this bug.