Users should be able to configure cluster specific tcp proxies

Filed by sregidor@redhat.com originally: https://bugzilla.redhat.com/show_bug.cgi?id=1937393

Description of problem:
In MTC installations where the source cluster is behind a proxy, we need to configure the variable stunnel_tcp_proxy in the controller cluster's migrationcontroller resource.

If there are more than one source cluster and they use different tcp proxies, we cannot use them without changing the migrationcontroller configuration every time we want to use a different source cluster.

Version-Release number of selected component (if applicable):
MTC 1.4.2

How reproducible:
Always

Steps to Reproduce:
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Actual results:
When we have several source clusters and they use different tcp proxies, we need to reconfigure the migrationcontroller resource every time we want to use a new source cluster.

Expected results:
Instead of configuring the stunnel_tcp_proxy variable in the migration cluster's migrationcontroller resource, we should configure it in the source cluster's migrationcontroller resource, so that we use the right tcp proxy for every source cluster without having to reconfigure the migrationcontroller resource every time.