XD-1305 File Sink now supports configurable modes

Originally the file sink only supported the append mode.
Now supports APPEND, REPLACE, FAIL IGNORE
XD-1294 Update xd reactor dependency

- Move the reactor Environment bean definition out of module common context
- Remove the 'spring-xd-extension-reactor' runtime dependency from spring-xd-dirt
- Add back 'spring-xd-extension-reactor' runtime dependency to reactor-syslog module

Add reactorEnv bean to syslog module

- Add the reactor environment bean to the reactor-syslog source
module so that the reactor environment is only loaded when this
module is deployed.