Can't specify --user-filter or other string-valued args

(found in v2.3rc2)

Something is wrong with the merging of invocation defaults and argument values: specifying ``--user-filter ".*@ensemble.com"`` produces:
```
2017-12-08 16:11:59 27327 CRITICAL main - Bad regular expression in user filter: ['.*@ensemble.com'] reason: cannot concatenate 'str' and 'list' objects
```
