Unnecessary errors from LookupDataAdapters that are disabled by default

Service startup generates several error messages with stacktraces from LookupDataAdapters that are disabled by default.

## Expected Behavior

Successful startup should not trigger error messages with stacktrace.

## Current Behavior

Threat Intelligence Lookup plugins are disabled by default and that generates errors in service startup.

```
2020-12-11T11:07:15.218+02:00 ERROR [LookupDataAdapter] Couldn't start data adapter
org.graylog.plugins.threatintel.tools.AdapterDisabledException: Spamhaus service is disabled, not starting (E)DROP adapter. To enable it please go to System / Configurations.
at org.graylog.plugins.threatintel.adapters.spamhaus.SpamhausEDROPDataAdapter.doStart(SpamhausEDROPDataAdapter.java:85) ~[?:?]
at org.graylog2.plugin.lookup.LookupDataAdapter.startUp(LookupDataAdapter.java:79) [graylog.jar:?]
at com.google.common.util.concurrent.AbstractIdleService$DelegateService$1.run(AbstractIdleService.java:62) [graylog.jar:?]
at com.google.common.util.concurrent.Callables$4.run(Callables.java:119) [graylog.jar:?]
at java.lang.Thread.run(Thread.java:834) [?:?]
2020-12-11T11:07:15.220+02:00 ERROR [LookupDataAdapter] Couldn't start data adapter
org.graylog.plugins.threatintel.tools.AdapterDisabledException: Abuse.ch service is disabled, not starting adapter. To enable it please go to System / Configurations.
at org.graylog.plugins.threatintel.adapters.abusech.AbuseChRansomAdapter.doStart(AbuseChRansomAdapter.java:96) ~[?:?]
at org.graylog2.plugin.lookup.LookupDataAdapter.startUp(LookupDataAdapter.java:79) [graylog.jar:?]
at com.google.common.util.concurrent.AbstractIdleService$DelegateService$1.run(AbstractIdleService.java:62) [graylog.jar:?]
at com.google.common.util.concurrent.Callables$4.run(Callables.java:119) [graylog.jar:?]
at java.lang.Thread.run(Thread.java:834) [?:?]
2020-12-11T11:07:15.219+02:00 ERROR [LookupDataAdapter] Couldn't start data adapter
org.graylog.plugins.threatintel.tools.AdapterDisabledException: Abuse.ch service is disabled, not starting adapter. To enable it please go to System / Configurations.
at org.graylog.plugins.threatintel.adapters.abusech.AbuseChRansomAdapter.doStart(AbuseChRansomAdapter.java:96) ~[?:?]
at org.graylog2.plugin.lookup.LookupDataAdapter.startUp(LookupDataAdapter.java:79) [graylog.jar:?]
at com.google.common.util.concurrent.AbstractIdleService$DelegateService$1.run(AbstractIdleService.java:62) [graylog.jar:?]
at com.google.common.util.concurrent.Callables$4.run(Callables.java:119) [graylog.jar:?]
at java.lang.Thread.run(Thread.java:834) [?:?]
2020-12-11T11:07:15.220+02:00 ERROR [LookupDataAdapter] Couldn't start data adapter
org.graylog.plugins.threatintel.tools.AdapterDisabledException: TOR service is disabled, not starting TOR exit addresses adapter. To enable it please go to System / Configurations.
at org.graylog.plugins.threatintel.adapters.tor.TorExitNodeDataAdapter.doStart(TorExitNodeDataAdapter.java:89) ~[?:?]
at org.graylog2.plugin.lookup.LookupDataAdapter.startUp(LookupDataAdapter.java:79) [graylog.jar:?]
at com.google.common.util.concurrent.AbstractIdleService$DelegateService$1.run(AbstractIdleService.java:62) [graylog.jar:?]
at com.google.common.util.concurrent.Callables$4.run(Callables.java:119) [graylog.jar:?]
at java.lang.Thread.run(Thread.java:834) [?:?]

```

## Possible Solution

Don't try to start adapters related to disabled plugin.

## Steps to Reproduce (for bugs)

1. Start Graylog service with default configurations
2. Check log

## Your Environment

* Graylog Version: 4.0.1
* Java Version: openjdk version "11.0.9" 2020-10-20 LTS