Fix int to string conversions

[hakman]

Go 1.15 introduces some new restrictions:
https://tip.golang.org/doc/go1.15#vet

# k8s.io/kops/pkg/instancegroups
pkg/instancegroups/rollingupdate_test.go:139:16: conversion from int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
FAIL	k8s.io/kops/pkg/instancegroups [build failed]

# k8s.io/kops/upup/pkg/fi/cloudup/awsup
upup/pkg/fi/cloudup/awsup/machine_types.go:61:30: conversion from int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
FAIL	k8s.io/kops/upup/pkg/fi/cloudup/awsup [build failed]

[k8s-ci-robot]

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[johngmyers]

/lgtm