Add descriptions to the network resource command documentation.

Change-Id: I547ffb48f8950311a5ee65d6b535846f2aca0efc

Update the "os availability zone list" command to support listing
network availability zones along with the currently listed compute
and volume availability zones. This adds the --network option to
the command in order to only list network availability zones. By
default, all availability zones are listed. The --long option
was also updated to include a "Zone Resource" column which is
applicable to network availability zones. Example zone resources
include "network" and "router".

If the Network API does not support listing availability zones
then a warning message will be issued when the --network option
is specified.

This support requires an updated release of the SDK in order
to pull in [1].

[1] https://bugs.launchpad.net/python-openstacksdk/+bug/1532274

Change-Id: I78811d659b793d9d2111ea54665d5fe7e4887264
Closes-Bug: #1534202

Update the "os availability zone list" command to support listing
volume availability zones along with the currently listed compute
availability zones. This adds a --compute and --volume option to
the command in order to select the availability zones to list. By
default, all availability zones are listed.

If the Block Storage API does not support listing availability
zones then an warning message will be issued.

Change-Id: I8159509a41bd1fb1b4e77fdbb512cf64a5ac11a9
Closes-Bug: #1532945

For consistency, use dashes instead of underscores.

Change-Id: I2da19b56952a8a9d172793ae211bdc58ddff4146