Fix startup error when not using any dns plugins

[yachub]

This issue has been copied from Jira at https://puppet.atlassian.net/browse/POOLER-228

With DNS plugin introduction in vmpooler 3.x, a bug was introduced in the manager where it fails to start if no dns gems are included in dns_configs. Manager should be allowed to run only with a dynamic-dns config. Example trace:

Error while creating dns plugin for pool debian-11-x86_64: uninitialized constant Vmpooler::PoolManager::Dns
docker-vmpooler-manager-1  | warning: thread "Ruby-0-Thread-3: /opt/vmpooler/bin/vmpooler:52" terminated with exception (report_on_exception is true):
docker-vmpooler-manager-1  | NameError: uninitialized constant Vmpooler::PoolManager::Dns
docker-vmpooler-manager-1  |       const_missing at org/jruby/RubyModule.java:4309
docker-vmpooler-manager-1  |   create_dns_object at /opt/vmpooler/lib/vmpooler/pool_manager.rb:1450
docker-vmpooler-manager-1  |            execute! at /opt/vmpooler/lib/vmpooler/pool_manager.rb:1703
docker-vmpooler-manager-1  |                each at org/jruby/RubyArray.java:1987
docker-vmpooler-manager-1  |            execute! at /opt/vmpooler/lib/vmpooler/pool_manager.rb:1669
docker-vmpooler-manager-1  |              <main> at /opt/vmpooler/bin/vmpooler:58