Introduce get_ports_attributes in OVSBridge

Introduce get_ports_attributes in OVSBridge

OVSBridge was inheriting db_list from BaseOVS, which was
returning the information of all the ports on the machine,
not only the ones belonging to the bridge.
The OVSNeutronAgent was using that method with the assumption
that ports were filtered by bridge.
To avoid confusion, this patch add a new method to OVSBridge
get_ports_attributes to query the info for all the ports
belonging to the bridge.
db_list is removed from BaseOVS since that method is already
available in ovsdb/api.py
ovs_lib methods that use db_list are refactored accordingly.

Co-Authored-By: Assaf Muller <amuller@redhat.com>
Change-Id: I2ce6d232744f48ba7fc0f824a7db32e3655bc2aa
Closes-Bug: 1473199