Limit Pecan version, fix Pandas usage

In order to bypass bug fixed in
https://review.openstack.org/#/c/129602/, we limit Pecan to version
before 0.8.

Also, let's fix get_measure() usage by passing datetime object directly
to get_measure() rather than random string. We validate them in the API
like the rest of timestamp objects.

Change-Id: Ia028a8cc7badc4e03cf27e3d146f26f989bfc540