Fix clippy errors on nightly (2021-09-29) (#691)

Most of the changes are for the new if-then-panic lint added in
https://github.com/rust-lang/rust-clippy/pull/7669.

control_plane/src/compute.rs

@@ -452,9 +452,7 @@ impl PostgresNode {
             .output()
             .expect("failed to execute whoami");
 
-        if !output.status.success() {
-            panic!("whoami failed");
-        }
+        assert!(output.status.success(), "whoami failed");
 
         String::from_utf8(output.stdout).unwrap().trim().to_string()
     }