Document and restrict the length of Environment Prefix to 8 chars.

[richbrowne]

As a part of the differentiated environments we did not enforce a length limit on the string for creating new environments. We need to set this limit to 8 characters and document it. The changes can be enforced in the driver code by:

1. Coding the diff env tool to only create environments with max length 8.
2. Throwing an exception in the f5-openstack-lbaasv2-driver if the length exceeds 8.

Also the limit needs to be documented in the Differentiated environments docs.

Agent Version

All versions

Operating System

N/A

OpenStack Release

Liberty and Mitaka