incorrect "Couldn't parse query: 'group by' (target_type=, unit=, server), 'sum by ()' and 'avg by (server)' cannot list the same tag keys " when using buckets

[Dieterbe]

`swift_proxy_server GET timing 200 object median group by server:dfvimeo|lvim avg by server avg over 10M"

-->

Couldn't parse query: 'group by' (target_type=, unit=, server), 'sum by ()' and 'avg by (server)' cannot list the same tag keys
Traceback (most recent call last):
File "/opt/graph-explorer/app.py", line 390, in render_graphs
query = Query(query)
File "/opt/graph-explorer/query.py", line 26, in __init__
self.parse(query_str)
File "/opt/graph-explorer/query.py", line 92, in parse
(', '.join(self['group_by'].keys()), ', '.join(self['sum_by'].keys()), ', '.join(self['avg_by'].keys())))
Exception: 'group by' (target_type=, unit=, server), 'sum by ()' and 'avg by (server)' cannot list the same tag keys 

when using buckets, this should be allowed. the first bucket catches 2 diff. values of server, the other one about 10. they should each avg by server, after doing the group by server with the buckets.

[Dieterbe]

fixed in 49560ea086811665bc13182f7e940588933f9fc8