XD-2235 Allow for 'security.basic.realm' to control the security realm

This is a minor fix, but without it, the realm will always default to null
Same here: realm -> `SpringXD` (what is derived from application.yml). Maybe you would want to set realm here:
```
classpath:org/springframework/xd/dirt/security/simpleSecurity.yml
```
which could test overriding values ;)