Navigating to the Master account overview in Multitenant throws null pointer exception

This has been tested in a local environment. The issue is @plan being nil because of the account being 'master'. Line:

https://github.com/3scale/porta/commit/320a96566749112f230936bca82a89448c57b3a9#diff-f7cc1233d38f7887aa8b04030ab0b8311a2939a222d36d27b636370314e90178L19-R19

To reproduce, in the local env open master-account, go to Accounts and click on Provider Name.