director-config.yml sallows allow availability_mode

[bottkars]

PCF 2.5 on Azure allows for Availability Zones OR availability sets. ( Availability Zones are not available in all regions )
therfore , availability mode is to be set.

In case of
availability_mode: availability_set
singleton_availability_zone:
name: ((singleton_availability_zone)) must be 'null:

In case of
availability_mode: availability_zone
singleton_availability_zone:
name: ((singleton_availability_zone)) must be one of zone-1,zone-2,zone-3