install: Move PUPPET_CLASSES env var to --puppet-classes argument.

`--no-init-db` is used to silence the need for `--hostname` and
`--email` arguments; it is a proxy for "this is not a frontend host."
We would ideally like to use `has_class` to know if the user's
provided puppet classes are include an `app_frontend`, and thus
`--hostname` and `--email` are required -- but doing that requires
several other steps, and we would like this feedback to be immediate.

We make the presence of `--puppet-classes` equivalent to
`--no-init-db`, since nearly every configuration with
`--puppet-classes` does not install both a database and a frontend,
which is what is required to initialize a database.

tools/setup/bootstrap-aws-installer

@@ -71,11 +71,8 @@ git clone "$REPO_URL" zulip -b "$BRANCH"
 git -C zulip checkout "$BRANCH"
 
 (
-    export PUPPET_CLASSES="$FULL_ROLES"
     export APT_OPTIONS="-o Dpkg::Options::=--force-confnew"
-    /root/zulip/scripts/setup/install \
-        --self-signed-cert \
-        --no-init-db
+    /root/zulip/scripts/setup/install --puppet-classes "$FULL_ROLES"
 )
 
 # Delete the ubuntu user