Config key for brookly-web-server temp dir
Fix BrooklynConfigKeys.BROOKLYN_WEB_SERVER_BASE_DIR

- Make it a TemplatedStringAttributeSensorAndConfigKey
- Add ConfigToAttributes.transform(managementContext, key)
  so can be used by things other than entities
- BrooklynWebServer calls ConfigToAttributes.transform for baseDir
Merge pull request #1219 from aledsage/feature/webdir-config

Config key for brookly-web-server temp dir