Clear state later in ceilometer driver

The previous version cleared self.state before update_from_datasource() was
finished using self.state.  This caused KeyError failures when the ceilometer
data stays the same.  This change clears self.state after
update_from_datasource() is finished using it.

Change-Id: I97df021b6036c5d80bc2c7f97f5dba8ebbd0dd38
Merge "Clear state later in ceilometer driver"