azurerm_role_assignment: fix import when use role name instead of id

[katbyte]

attempt to fix:

        --- PASS: TestAccAzureRMRoleAssignment/basic/custom (42.86s)
        --- PASS: TestAccAzureRMRoleAssignment/basic/builtin (32.28s)
        --- FAIL: TestAccAzureRMRoleAssignment/basic/roleName (23.27s)
        	testing.go:538: 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=20) "role_definition_name": (string) (len=20) "Log Analytics Reader"
        		}

[tombuildsstuff]

LGTM 👍

[tombuildsstuff]

Tests pass:

[ghost]

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