Reference QUOTA OptGoup names in lowercase

To have consistent option group name format across projects,
oslo.config now normalizes all non lowercase group names to lowercase
when loading conf files.
Thus conf files are backwards compatible but option group references
in the code must now use lowercase before we update oslo.config.

This patch replaces all 'QUOTA' references to 'quota'

Change-Id: I74c2a35aea7a52f9586eb598fe52d2cecb1851ef
Merge "Reference QUOTA OptGoup names in lowercase"