cloud_storage: consistent handling of booleans in topic config properties

[jcsp]

Setting remote.write to True is accepted but does not take effect:

root@231067b69493:/# /home/vectorized/redpanda/vbuild/debug/clang/dist/local/redpanda/bin/rpk --brokers docker-rp-1:9092,docker-rp-3:9092,docker-rp-2:9092 topic alter-config test-si-topic-with-retention-altered --set redpanda.remote.write=True 
TOPIC                                 STATUS
test-si-topic-with-retention-altered  OK

root@231067b69493:/# /home/vectorized/redpanda/vbuild/debug/clang/dist/local/redpanda/bin/rpk --brokers docker-rp-1:9092,docker-rp-3:9092,docker-rp-2:9092 topic describe test-si-topic-with-retention-altered
SUMMARY
=======
NAME        test-si-topic-with-retention-altered
PARTITIONS  1
REPLICAS    1

CONFIGS
=======
KEY                     VALUE                          SOURCE
...
redpanda.remote.write   false                          DYNAMIC_TOPIC_CONFIG

We should make all our boolean inputs take as consistent set of value values. I think we have to support "yes" and "no" for backward compat as those are already in there. We should also accept case-insensitive true/false strings.

JIRA Link: CORE-1044

[jcsp]

(in the process of doing this, clean up places using explicit string_switch with true/false to use the same common helper