common: str_map: have 'get_str_map' only handling plain-text

common: str_map: have 'get_str_map' only handling plain-text

'get_str_map()' used to handle both JSON and plain-text.  In fact it
would try parsing the map as JSON on a first try and then fallback to
plain-text if it failed.  Altough useful this would pose a big issue
when we attempted to parse some values, tha we knew to be plain-text,
that had some meaning in JSON -- e.g., 'false' or 'true'.  In such case
the JSON parser would spit out an error, stating it had been able to
parse the JSON but didn't expected the type, which in fairness is
acceptable.

In its stead we now have two functions: 'get_str_map()' will only handle
plain-text, whereas 'get_json_str_map()' will keep the previous
behavior, attempting to parse a JSON string and falling back to
'get_str_map()' should it fail.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>