Define $user referenced in db.pp

In db.pp, in the exec 'ceilometer dbsync' ressource the user
is supposed to be $::ceilometer::params::user, but it was
not defined.

(cherry picked from commit e6c191194312935ea01c97ae667bf3ea1a912f81)

Conflicts:
	spec/classes/ceilometer_db_spec.rb

Change-Id: I888be405f418932c0e46ce8e82486755d36fc7bf