This commit makes 'log' optional.
'log' attribute of each command class does not exist
when the class is defined because 'log' is now setup
dynamically when a class is instantiated. Instead log_method
looks for a logger from a decorating method.
compute.v2.server is changed in this commit as an example.
Change-Id: Ic4d128f8e027d3b8e6f884f31369e9085c0f0871
Partial-Bug: #1532294