fix commitment move in overcommitted AZ

[majewsky]

I already tried to fix this in 5f75541332d84fc69f669f7eef8257673b10e1a4, but it did not go all the way. We saw this in a debug log:

DEBUG: checking CanMoveExistingCommitment in compute/ram/eu-de-1d: resourceID = 3296 -> 187489, amount = 412252
DEBUG: CanAcceptCommitmentChanges: forbidden by commitment target 24429662 > usage 23889216 in resourceID = 3296

The problem here is that committed > usage was evaluated in the project where committed shrinks, even though it is only relevant in the project where committed expands.

Checklist:

• [ ] If this PR is about a plugin, I tested the plugin against an OpenStack cluster.
• [ ] I updated the documentation to describe the semantical or interface changes I introduced.

[coveralls]

coverage: 79.354% (-0.01%) from 79.365% when pulling 8cf4b6464ada6417be4d312aa664e786cf6d4c7c on fix-commitment-move-in-overcommitted-az into d34239c1ca51b5f3bd33979ba1b59ae19b37b600 on master.