Remove message signing in API plugin (after discussion with ceilometer team).
Signing doesn't prevent replay attacks. Use HTTPS to not reinvent the wheel.
Remove ceilometer-pollster directory (it has been merged with Ceilometer).

Change-Id: Id26497a7655c69b367aeee959d0078495879b1cf
Merge "Remove message signing in API plugin (after discussion with ceilometer team). Signing doesn't prevent replay attacks. Use HTTPS to not reinvent the wheel. Remove ceilometer-pollster directory (it has been merged with Ceilometer)."