Rework cpuShares into cpuQuota

cpuShares is the relative weight wrt other apps. This is used when
there is contention for CPU. If we want this, maybe we implement
a UI where we show all the apps and let the user re-order them.
As it stands, it is confusing.

cpuQuota is a more straightforward "hard limit" of the CPU% that you
want the app to consume.

Can be tested with : stress -c 8 -t 20s

src/test/apps-test.js

@@ -302,7 +302,7 @@ describe('Apps', function () {
                 manifest: Object.assign({}, app.manifest, { version: '0.2.0' }),
                 accessRestriction: '',
                 memoryLimit: 1337,
-                cpuShares: 102,
+                cpuQuota: 79,
                 operators: { users: [ 'someid' ] }
             };
 
@@ -313,7 +313,7 @@ describe('Apps', function () {
             expect(newApp.manifest.version).to.be('0.2.0');
             expect(newApp.accessRestriction).to.be('');
             expect(newApp.memoryLimit).to.be(1337);
-            expect(newApp.cpuShares).to.be(102);
+            expect(newApp.cpuQuota).to.be(79);
         });
 
         it('update of nonexisting app fails', async function () {