Depending on python-swiftclient>=1.6

Now we're depending on python-swiftclient>=1.6 since the parameter
'headers' was introduced since python-swiftclient 1.6. This is a
regression issue introduced by patch:
https://review.openstack.org/#/c/48913/

get_object(container, obj, resp_chunk_size=None, query_string=None,
response_dict=None, headers=None)

Here is the commit of adding param 'headers' for Swift client, see:
https://github.com/openstack/python-swiftclient/commit/d34f12614820416aebe5e91eab9726b556d1f973

Closes-Bug: #1276033

Change-Id: Ibebf1d1ac8df9f35a7d5b5745eddfcdb00a036df
Merge "Depending on python-swiftclient>=1.6"