Unable to enforce string on input

```

self.template.add_resource(VPCPeeringConnection(
title,
VpcId=peering_config['VpcId'],
PeerVpcId=peering_config['TargetVpcId'],
PeerOwnerId=peering_config['TargetAccountId'],
PeerRoleArn=peering_config['TargetRoleArn'],
Tags=Tags(
Name=title
)
))
```

as account ID is 1234567890 format. the Account ID keep written as Number instead of String. this cause cloudformation complain about the value
```

[2018-03-05 16:36:47] - sceptre.stack - 2018-03-05T03:36:47+00:00 prod/cross-account-vpc-peering ProdVpcToDev AWS::EC2::VPCPeeringConnection CREATE_FAILED Invalid value '1.234567890E10' for peerOwnerId.

```

even if i use str() to convert it. it still written as number in yaml output