Default REAPER_HTTP_MANAGEMENT_ENABLE environment variable

[Miles-Garnsey]

Add default REAPER_HTTP_MANAGEMENT_ENABLE="false" value to docker environment to ensure proper startup when not explicitly set.

At present, the docker container fails to start because this value is not defined anywhere and Dropwizards throws:

docker run australia-southeast1-docker.pkg.dev/k8ssandra/miles/cassandra-reaper:http-management3
WARN  [2023-10-17 00:46:54,064] com.datastax.driver.core.utils.UUIDs: PID returned through native call was 0, JNR versions incompatible?  Falling back to JMX.
io.dropwizard.configuration.UndefinedEnvironmentVariableException: Cannot resolve variable 'REAPER_HTTP_MANAGEMENT_ENABLE' (enableSubstitutionInVariables=false).
        at io.dropwizard.configuration.EnvironmentVariableSubstitutor.substitute(EnvironmentVariableSubstitutor.java:37)
        at org.apache.commons.text.StringSubstitutor.replace(StringSubstitutor.java:893)
        at io.dropwizard.configuration.SubstitutingSourceProvider.open(SubstitutingSourceProvider.java:41)
        at io.dropwizard.configuration.BaseConfigurationFactory.build(BaseConfigurationFactory.java:81)
        at io.dropwizard.cli.ConfiguredCommand.parseConfiguration(ConfiguredCommand.java:137)
        at io.dropwizard.cli.ConfiguredCommand.run(ConfiguredCommand.java:85)
        at io.dropwizard.cli.Cli.run(Cli.java:78)
        at io.dropwizard.Application.run(Application.java:94)
        at io.cassandrareaper.ReaperApplication.main(ReaperApplication.java:99)

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