azurestack_public_ip: correct import & read idle_timeout_in_minutes

[katbyte]

fixes the following import error:

------- Stdout: -------
=== RUN   TestAccAzureStackPublicIpStatic_idleTimeout
--- FAIL: TestAccAzureStackPublicIpStatic_idleTimeout (71.27s)
	testing.go:513: Step 1 error: ImportStateVerify attributes not equivalent. Difference is shown below. Top is actual, bottom is expected.
		
		(map[string]string) {
		}
		
		
		(map[string]string) (len=1) {
		 (string) (len=23) "idle_timeout_in_minutes": (string) (len=2) "30"
		}
		
FAIL

(and its nice if changes to the value are read back in)

[katbyte]

Tests all pass:

[tombuildsstuff]

LGTM

[ghost]

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