Use consul-template if-statement to check if the listenstore is up

[alastair]

We want a consul setting that we can use to shut down the listenstore
when we need to do maintenance.
We initially used a python if statement here, expecting to be able to
render the config file as

if False:
    SQLALCHEMY_TIMESCALE_URI = """postgresql:// ..."""
else:
    SQLALCHEMY_TIMESCALE_URI = ""

However this doesn't work when the service disappears from consul (for
example if we shut down the server). The config file ends up as

if False:
    SQLALCHEMY_TIMESCALE_URI = "SERVICEDOESNOTEXIST_timescale-listenbrainz"
else:
    SQLALCHEMY_TIMESCALE_URI = ""

and our startup script (run-lb-command) fails because it finds the sentinel
SERVICEDOESNOTEXIST in the config and refuses to continue.

Instead, use a consul-template {{if}} to render this config depending on
the listenstore_up config item.

This usage duplicates the (printf / env ()) syntax used in our KEY
template as it's not possible to call a template inside an if statement
or a variable declaration

This requires an update to the consul settings for LB to switch to using a json boolean (true/false) instead of a string with a python True/False at the same time that we deploy this.

[amCap1712]

LGTM. Thanks!