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.

scripts/lib/install

@@ -10,12 +10,14 @@ Usage:
 
 Options:
   --hostname=zulip.example.com
-      The user-accessible domain name for this Zulip server, i.e., what users will type
-      in their web browser.  Required, unless --no-init-db is set and --certbot is not.
+      The user-accessible domain name for this Zulip server, i.e.,
+      what users will type in their web browser.  Required, unless
+      --no-init-db or --puppet-classes is set, and --certbot is not.
+
   --email=zulip-admin@example.com
-      The email address of the person or team who should get support and error emails
-      from this Zulip server.  Required, unless --no-init-db is set and --certbot is
-      not.
+      The email address of the person or team who should get support
+      and error emails from this Zulip server.  Required, unless
+      --no-init-db or --puppet-classes is set and --certbot is not.
 
   --certbot
       Obtains a free SSL certificate for the server using Certbot,
@@ -37,9 +39,13 @@ Options:
   --postgresql-missing-dictionaries
       Set postgresql.missing_dictionaries, which alters the initial database.  Use with
       cloud managed databases like RDS.  Conflicts with --no-overwrite-settings.
+
   --no-init-db
       Does not do any database initialization; use when you already have a Zulip
       database.
+  --puppet-classes
+      Comma-separated list of Puppet classes to install; defaults to
+      'zulip:::profile::standalone'.  Implies --no-init-db.
 
   --no-overwrite-settings
       Preserve existing `/etc/zulip` configuration files.
@@ -119,6 +125,12 @@ while true; do
             NO_INIT_DB=1
             shift
             ;;
+        --puppet-classes)
+            PUPPET_CLASSES="$2"
+            NO_INIT_DB=1
+            shift
+            shift
+            ;;
 
         --no-overwrite-settings)
             NO_OVERWRITE_SETTINGS=1