Remove monitoring user from documentation

Recent commit e3db2693b50d9d950adc6119135403eb235335ab created the
'ceilometer' user. A 'monitoring' user is left over and being created
but it isn't being used. This patch removes a reference to it in the
documentation.

Change-Id: I503b7e3d2d6484126f8e798a36fb47f6f717e17a