Allow updating fixed_ips quotas in quota-class-update (v2 shell only)

The quota-class-update command wasn't allowing updating quotas for
fixed_ips which is a supported key for the os-quota-class-sets API.

This change adds the support to update qoutas for fixed_ips and updates
the existing test to be more robust in (a) the quota keys it's testing
and (b) the request body it's asserting.

This is a v2-only change since the os-quota-class-sets API was removed
from the nova v3 API in Icehouse.

Closes-Bug: #1330571

Change-Id: Ifcca6d00551619b993b96f05134980a4ca2f2acf