droplets: Make assert_droplet_does_not_exist take droplet_name as argument.
droplets: Make assert_droplet_does_not_exist take droplet_name as argument.

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