Add server_name_prefix semi-random naming with some known prefix

Perhaps this will be useful for others so I thought I'd share my changes. We have several groups using our Openstack infra and sometimes people leave old instances lying around and it would be useful if we could prefix the generated server names with the relevant project. This PR implements a generated name consisting of

    <value of server_name_prefix>-<random 8 char string>

the intention being we can prefix the random string with the project being tested. For example

    myspecialproject-asdfghjk

I have also added some tests for this. Hopefully this is something others will consider useful - any constructive feedback would be appreciated.