Use CEILOMETER_TOKEN instead of ADMIN_TOKEN for ceilometer secret key

The ADMIN_TOKEN replaceable is already used to reference the keystone
secret token, using it again for ceilometer might confuse readers. Using
CEILOMETER_TOKEN should make things clearer.

Change-Id: Id1cd9ab474f0b94a7889a03afb00c1f645d5217d
Merge "Use CEILOMETER_TOKEN instead of ADMIN_TOKEN for ceilometer secret key"