Allow to use schedd which is not in the rest configuration list

This will allow us to test schedd or changed puppet configuration using main interface. Like now :

On global pool waiting 2 schedds : vocms095 and vocms096. To check them need to have full CrabServer installation or change rest configuration for preprod or dev. To change pool, schedds on rest configuration is not good, we saw a lot of issues with this change https://github.com/dmwm/CRABServer/issues/4336

Friday(09.19) ~5PM vocms0109 went down because puppet configuration started running and messed up whole schedd (ELOG filled). It was take out from rest configuration and condor was stopped. Whenever it will be fixed, it will require testing before going to prod rest configuration.

We talked several times to leave Hammercloud running with submission of minimal jobs and test schedd changes, if we want to change to newer version, or do modifications on schedd. This fix/feature will allow to specify schedd name on which change was made and to make sure that it did not break anything.

Addon in Client : https://github.com/juztas/CRABClient/commit/42d602e35f452c8067f94f193229823c3659804c

@jamesletts @mmascher @bbockelm