* Compute V2: implement server OS-SRV-USG extension

Add the new "serverusage" package with basic structure and UnmarshalJSON
method.
Add unit test and update TestServersWithExtensionsCreateDestroy aceptance
test.
Add documentation example.

* Compute V2: fix OS-SRV-USG documentation

Use "computeClient" variable name instead of fake.Serviceclient().

* Compute V2: update OS-SRV-USG acc test

Fix checks for extendedServer LaunchedAt, TerminatedAt fields.