Remove redundant 'availability_zone' config options.

Remove redundant 'availability_zone' config options.

There are two availability_zone related config options now:
  node_availability_zone and storage_availability_zone
While volume API uses node_availability_zone, the scheduler uses
the other, which causes failure in scheduler when these two options
have different values.

This patch removes 'node_availability_zone' so that end user
can specify 'availability zone' when create volumes using
Simple scheduler.

Partially implement blueprint implement-availability-zones

Change-Id: Ia2f0710908495ec64dacd2ee9bd10d7f8cd415df