droplets: Rename fork_exists to assert_user_forked_zulip_server_repo.
droplets: Rename fork_exists to assert_user_forked_zulip_server_repo.

punchagan/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,punchagan/zulip,andersk/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip