Monetization (MON-190): MonetizationModule - updated cache ttl

This doesn't reflect the latest changes to the ticket.  We want the TTL to be between 60 and 120 minutes.  

If we go with a base +- variance then then the call should be 

```
$this->cacheTtl = mt_rand( ( self::CACHE_TTL - $range ), ( self::CACHE_TTL + $range ) );
```
The variance shouldn't depend on the base TTL, so let's define 2 constants, either
```
CACHE_BASE_TTL = 5400
CACHE_VARIANCE_TTL = 1800
```
or
```
CACHE_MIN_TTL = 3600
CACHE_MAX_TTL = 7200
```

The latter has the advantage of not requiring any calculation, the former can be tuned by changing only 1 constant.