Replace get_all_neutron_bridges by get_deletable_bridges

Replace get_all_neutron_bridges by get_deletable_bridges

Currently LinuxBridge L2-agent[1] defines get_all_neutron_bridges which
lists all bridges managed by the agent including user-defined ones BUT
this method is only used by neutron-linuxbridge-cleanup[2] in order to
remove empty non-user-defined bridges returned get_all_neutron_bridges.

This change replaces this method by get_deletable_bridges which returns
bridges managed by LB agent which are "deletable" (ie: non user-defined).
This allows to simplify code and move the computation of "deletable"
bridges to LB agent code.

[1] neutron.plugins.ml2.drivers.linuxbridge.agent.linuxbridge_neutron_agent
[2] neutron.cmd.linuxbridge_cleanup

Partial-Bug: #1514548
Change-Id: I2f8a9a6982ce0ce77187ae4b7a58b3ef752aab34