Updates to the VMS policy string

Fixes OS-113

The vms policy string is now of the form:

    ;$key=$value;

for a list of key,value pairs, and the key should be presented in
lexigraphical order. By default the keys are flavor, name and
tenant:

    ;flavor=$flavor;;name=$instance_name;;tenant=$project_id;

Change-Id: I13704254dbc605c9b6a4bc84fc7ccc455eb4d29b
Merge "Updates to the VMS policy string" into stable/essex