VAF plugin

Addition of the `vaf-setup.py` plugin. Configuration example in the `user-data` file:

```ini
[vaf-setup]
# can be master or slave
node_type=master
# can be pool_users or alice_ldap
auth_method=alice_ldap
# num of pool accounts to create (if auth_method==pool_users)
#num_pool_accounts=50
# which VAF client settings to use
client_settings=alice
# for ALICE: VAF storage specification
alice_storage=ALICE::Torino::SE
```

If `node_type=master` it configures [sshcertauth](https://github.com/dberzano/sshcertauth).

Options `client_settings` and `node_type` are not required. It is for instance possible to enable the sole ALICE AliEn LDAP authentication without sshcertauth by specifying only:

```ini
[vaf-setup]
auth_method=alice_ldap
```