arxiv: fix database search with prefix

• Adds an instance wide search parameter prefix to perform the database search.

• NOTE: for the search to be performed in the database, the arXiv field prefix for the corresponding instance config file should be added to invenio.cfg as ARXIV_SEARCH_PREFIX (e.g. ARXIV_SEARCH_PREFIX = u'035__a:oai:arXiv.org:').

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>