Stop using quota reservations on base controller

Stop using quota reservations on base controller

The reservation engine is subject to failures due to concurrency;
the switch to pymysql is likely to also have a part in observed
failures. While no gate failures have been observed so far, this
is a time bomb waiting to explode and must be addressed.

For this reason this patch acts conservatively by ensuring the
API controllers do not use anymore reservation. The code for
reservation management is preserved, and will wired again on the
controller when these issues are sorted.

The devref for neutron quotas is updated accordingly as a part
of this patch.

Related bug: #1486134

Change-Id: I2a95fef0fdf64ef8781bef99be0fdc743346c17a