CIMI: Added 'created' attribute to MachineTemplate

- New dependency: dm-timestamps
CIMI: Increased DataMapper 'string' length limit for MachineTemplate

In some cases 50 characters limit for 'string' property is not
enough (like when client have 'long' URL).
Core: Fixed timestamps and make be_id not required

The 'be_id' property should not be required globally,
since entities like MachineTemplate or VolumeConfig does
not have 'real' backend 'id' and they just exist in persistence
layer
CIMI: Replace save! with save in datamapper

For unknown reason the 'save!' method does not
work properly when using 'dm-timestamps'.

However there is no reason to use exlamation mark
suffix anyway.