Change subdomain strategy in config.yml to reference parameters.yml

[StephenGillCoop]

so currently in config.yml we have:

tahoe_multi_tenancy:
    account_prefix: %app_name%
    subdomain_strategy: fixed

change it something like:

tahoe_multi_tenancy:
    account_prefix: %app_name%
    subdomain_strategy: %multi_tenancy_subdomain_strategy%

and add the following to parameters.yml.dist:

parameters:
    multi_tenancy_subdomain_strategy: fixed