app: generate multiple strings in the generateUnitQuotaItems function

Related to #382.
app: fix generateUnitQuotaItems function

Related to #382.
app: implement partial quota support for units

Now AddUnits validates quotas. I will change how quota package behaves.

Related to #382.
quota: simplify the Reserve function

I will improve the QuotaExceeded error :-)

Related to #382.
quota: introduce type QuotaExceededError
quota: reduce the number of items in the Release safety test

I'm getting tired of this.
quota: use QuotaExceededError type

Related to #382.
app: fix tests related to quota package

Handling the recent changes in the Reserve function and the
QuotaExceededError type.

Related to #382.
api: fix quota related tests

Related to #382.
app: setting units-per-app quota to the value - 1

The provisioner will always allocate one unit in the Provision method,
so we need to subtract the value 1 from the limit.

Related to #382.