upgrade-zulip-stage-2: Added argument to skip purging old deployments.
upgrade-zulip-stage-2: Added argument to skip purging old deployments.

This makes it possible to add --skip-purge-old-deployments in the
deploy_options section of /etc/zulip/zulip.conf, and control whether
old deployments are purged automatically on a system.

We still need to do https://github.com/zulip/zulip/issues/10534 and
probably also to add these arguments to be directly passed into
upgrade-zulip, but that can wait for future work.

Fixes #10946.

andersk/zulip,dhcrzf/zulip,timabbott/zulip,kou/zulip,kou/zulip,hackerkid/zulip,tommyip/zulip,hackerkid/zulip,hackerkid/zulip,timabbott/zulip,hackerkid/zulip,zulip/zulip,timabbott/zulip,punchagan/zulip,tommyip/zulip,rishig/zulip,shubhamdhama/zulip,zulip/zulip,rishig/zulip,tommyip/zulip,shubhamdhama/zulip,eeshangarg/zulip,shubhamdhama/zulip,tommyip/zulip,brainwane/zulip,brainwane/zulip,hackerkid/zulip,synicalsyntax/zulip,rishig/zulip,punchagan/zulip,timabbott/zulip,rht/zulip,rht/zulip,punchagan/zulip,andersk/zulip,showell/zulip,dhcrzf/zulip,dhcrzf/zulip,hackerkid/zulip,brainwane/zulip,rishig/zulip,timabbott/zulip,shubhamdhama/zulip,punchagan/zulip,kou/zulip,synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,tommyip/zulip,andersk/zulip,shubhamdhama/zulip,brainwane/zulip,synicalsyntax/zulip,timabbott/zulip,showell/zulip,tommyip/zulip,eeshangarg/zulip,punchagan/zulip,dhcrzf/zulip,showell/zulip,showell/zulip,kou/zulip,brainwane/zulip,kou/zulip,andersk/zulip,rht/zulip,rishig/zulip,rht/zulip,brainwane/zulip,eeshangarg/zulip,showell/zulip,synicalsyntax/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,dhcrzf/zulip,zulip/zulip,rht/zulip,synicalsyntax/zulip,showell/zulip,zulip/zulip,dhcrzf/zulip,zulip/zulip,andersk/zulip,rishig/zulip,shubhamdhama/zulip,timabbott/zulip,eeshangarg/zulip,rishig/zulip,punchagan/zulip,rht/zulip,tommyip/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,synicalsyntax/zulip,zulip/zulip,rht/zulip,brainwane/zulip,synicalsyntax/zulip,shubhamdhama/zulip,showell/zulip,dhcrzf/zulip