Add uuid to temporary tenant names

Currently Rally names temporary tenants created before running scenarios as
"tenant_0", "tenant_1" etc., which leads to a name conflict while benchmarking
the same cloud from several instances of Rally simultaneously. This patch adds
uuid usage in tenant names to solve this issue. It also changes a bit the names
used for temporary users to make them easier to match with the corresponding
temporary tenants.

Change-Id: I28bc78f3e4040345c82da6d2cea1ffcb507dc821
Closes-Bug: #1254331