Requests with empty body failed to parse

[Ekaterina Chernova]

If the request has application/json murano client parses body without checking it.

Need to add this missed check

[Radek Pospisil (radek-pospisil)]

Is the "empty body" of request or response ?

Empty body of the *response* has to be also handled:

* $ murano environment-create dummy
+----------------------------------+-------+---------------------+---------------------+
| ID | Name | Created | Updated |
+----------------------------------+-------+---------------------+---------------------+
| db98819a639b4235b401763c3f8c3d81 | dummy | 2015-02-25T09:21:38 | 2015-02-25T09:21:38 |
+----------------------------------+-------+---------------------+---------------------+

* $ murano service-show db98819a639b4235b401763c3f8c3d81 /
'list' object has no attribute 'items'

The reason is that muranoclient/common/http.py json_request(..) return None on the empty list of env's services.