Make Impl of mongodb and sqlalchemy consistent

SqlAlchemy connection inherits from base.Connection, which doesn't have session
parameter in its function get_resources(). And session of get_resources() in
SqlAlchemy is not used and it can be got from self.session. So remove it to make
them consistent.

Change-Id: I85d5094d53af3de40645fa828d73d1dbc2677f0b
Signed-off-by: Shane Wang <shane.wang@intel.com>
sqlalchemy GET /v1/resources returns too much data

Meters data needs to match mongodb implementation.
The resources returned has meter attribute nested and
those results are each and every row of the meters for
that resource as well as all fields of each row. If a
resource has tens of thousands of meters associated to
it, the eager loading will load all of those rows and
return all that data in the json result. Instead, it
should match the mongodb implementation which simply
returns a distinct list of unique {'counter_name':
value, 'counter_type': value, 'counter_unit': value}.