glpi: Improve SQL count on Dashboard and computer's list

Replace sqlalchemy .count() by func.count() who improve speed

@see: http://docs.sqlalchemy.org/en/rel_0_7/orm/query.html

count()
    Return a count of rows this Query would return.

    This generates the SQL for this Query as follows:

    SELECT count(1) AS count_1 FROM (
                SELECT <rest of query follows...>
            ) AS anon_1

It's ugly !

Use of func.count() make a real count without a subquery.

From the same documentation page:

For fine grained control over specific columns to count, to skip the
usage of a subquery or otherwise control of the FROM clause, or to use
other aggregate functions, use func expressions in conjunction with
query().