xenapi: Remove vestigial `compile_metrics` code

`compile_metrics` is no longer called as it was superceded by:

    https://github.com/openstack/nova/commit/8887f10c66bca248f289db8f834ae8f36f9a03a1

Fixes bug 1194990

Change-Id: Ia651ac69fc38ed3f5cb29562556f7065caba225a
Break out compute API unit tests

As we convert things to objects, it'll be easier to convert our compute
API tests to be true unit tests if they are in their own class.  This
also allows us to move away from the hacks needed to simulate 2 DBs in
order to test ComputeCellsAPI().

There are currently a handful of tests that are true unit tests and
require no DB access.  These are now moved, subclassing from
test.NoDBTestCase.

Related to blueprint unified-object-model

Change-Id: I72f4f6ada8d0586e813e551a1b361bcebb16ee66
Break out Compute Manager unit tests

As we convert things to objects, it'll be easier to convert our compute
Manager tests to be true unit tests if they are in their own class.

There are currently a handful of tests that are true unit tests and
require no DB access.  These are now moved, subclassing from
test.NoDBTestCase.

Related to blueprint unified-object-model

Change-Id: I5e7ca3a94c32fff268e8b329f165198c873756e1
Merge "xenapi: Remove vestigial `compile_metrics` code"
Merge "Break out compute API unit tests"
Merge "Break out Compute Manager unit tests"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp