add tests for config.go and resolve a few config bugs

[cyrilst]

I love tests. These ones discovered a few bugs:

• in etcd and ssh subconfigs, overrides would crash sshproxy at config load
• overrides of intervals did not properly check if the interval values are strings before parsing them
• LoadAllDestsFromConfig did not expand destination nodesets

[github-actions]

File	Coverage
All files	16%	✅
cmd/sshproxy-dumpd/sshproxy-dumpd.go	0%	✅
cmd/sshproxy-replay/sshproxy-replay.go	0%	✅
cmd/sshproxy/commands.go	0%	✅
cmd/sshproxy/recorder.go	0%	✅
cmd/sshproxy/sshproxy.go	0%	✅
cmd/sshproxyctl/sshproxyctl.go	0%	✅
pkg/nodesets/nodesets.go	0%	✅
pkg/record/record.go	0%	✅
pkg/utils/config.go	100%	✅
pkg/utils/configv1.go	100%	✅
pkg/utils/etcd.go	0%	✅
pkg/utils/logging.go	0%	✅
pkg/utils/route.go	9%	✅
pkg/utils/utils.go	92%	✅

Minimum allowed coverage is 0%

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