The 2nd get_db_session() parameter should have a default

[Muharem Hrnjadovic]

Please see: openquake/db/alchemy/db_utils.py, line 72

<mattiab> al-maisan: while reviewing https://github.com/gem/openquake/pull/373 lars noticed that there is just a oq_get4gem and there aren't reader/writer roles and hence get_db_session does not work unless a env variable is set
<mattiab> al-maisan: should I add reader/writer roles or is the single oq_ged4gem user intentional?
<al-maisan> mattiab: yeah .. we could do get_db_session("get4gem", "")
<al-maisan> I was thinking of assigning a default ("writer") to the second param
<al-maisan> sorry, the default should be: ""
<al-maisan> in which case get_db_session("get4gem") suffices
<al-maisan> what do you think?
<mattiab> LGTM