integration tests don't work on openstack default flavors

When running the integration tests aginast and openstack provider, the m1.tiny instance is chosen. This happens because we specify 'mem=128' in our integration tests for postgresql and landscape-server.

https://pastebin.canonical.com/148781/

+----+-----------+-----------+------+-----------+------+-------+-------------+-----------+
| ID | Name | Memory_MB | Disk | Ephemeral | Swap | VCPUs | RXTX_Factor | Is_Public |
+----+-----------+-----------+------+-----------+------+-------+-------------+-----------+
| 1 | m1.tiny | 512 | 1 | 0 | | 1 | 1.0 | True |
| 2 | m1.small | 2048 | 20 | 0 | | 1 | 1.0 | True |
| 3 | m1.medium | 4096 | 40 | 0 | | 2 | 1.0 | True |
| 4 | m1.large | 8192 | 80 | 0 | | 4 | 1.0 | True |
| 5 | m1.xlarge | 16384 | 160 | 0 | | 8 | 1.0 | True |
+----+-----------+-----------+------+-----------+------+-------+-------------+-----------+

1) Our bundle by default uses 2G for these constraints (in the charm store), I think we should go with that, for consistency sake.

2) I have heard there was some fear that the local provider would not work with larger mem requirements, but AFAIK, the local provider does not consider constraints, since... LXC