Stylistic improvement of compute.api.API.update()

The previous implementation of compute.api.API.update() method included
a line of code where the '_' identifier was used to hold a throw-away
variable. Such a usage of underscores is generally not a good style and,
more importantly, it can possibly lead to a name conflict with the '_'
function imported as 'from nova.openstack.common.gettextutils import _'
in several modules of the system.

The present patch fixes the above, as well as some indent issues in the code.

Change-Id: I7a904109f4809fd677ded06b7eea3205c38a369b
Merge "Stylistic improvement of compute.api.API.update()"