process_fts_updates: Fix docker-zulip support.
process_fts_updates: Fix docker-zulip support.

In the series of migrations to this tool's configuration to support
specifying an arbitrary database name
(e.g. c17f502bb032385fc6d855b591544609cb179ae5), we broke support for
running process_fts_updates on the application server, connected to a
remote database server. That workflow is used by docker-zulip and
presumably other settings like Amazon RDS.

The fix is to import the Zulip virtualenv (if available) when running
on an application server.  This is better than just supporting this
case, since both docker-zulip and an Amazon RDS database are setting
where it would be inconvenient to run process-fts-updates directly on
the database server. (In the former case, because we want to avoid
having a strong version dependency on the postgres container).

Details are available in this conversation:
https://chat.zulip.org/#narrow/stream/49-development-help/topic/Logic.20in.20process_fts_updates.20seems.20to.20be.20broken/near/1251894

Thanks to Erik Tews for reporting and help in debugging this issue.

zulip/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,kou/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,rht/zulip,kou/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,rht/zulip