Allow to specify policy by name in firewall-update

Currently only firewall policy ids are supported in firewall-update,
this change allows to use firewall policy names in firewall-update:

  neutron firewall-update my-firewall --policy my-new-policy

DocImpact
Change-Id: I84ad97fc7f1f2e81cad98497d4d983606fa03077
Merge "Allow to specify policy by name in firewall-update"