Avoid use of replicaSet argument in pymongo.Connection() call 

On RedHat 5 EPL package for pymongo module (version 1.9) does not support
pymongo.Connection() call with replicaSet argument. Doing the replicaset name
check manually after the connection acquired is functionally equivalent for
check_replica_primary()

```bash
> ./check_mongodb.py -H localhost -A replica_primary -P 27017 --replicaset "tst_set03"
CRITICAL - General MongoDB Error: __init__() got an unexpected keyword argument  'replicaSet'
```