istioctl install: validate --revision param

[shamsher31]

Found during #28745

Before:
The existing validation was wrong which does not check for invalid --revision.

$ ./out/linux_amd64/istioctl install -r "1.8:" --set hub=gcr.io/istio-testing -d manifests/
Error: invalid revision specified:
name part must consist of alphanumeric characters, '-', '_' or '.', and must start and end with an alphanumeric character (e.g. 'MyName',  or 'my.name',  or '123-abc', regex used for validation is '([A-Za-z0-9][-A-Za-z0-9_.]*)?[A-Za-z0-9]')

$ ./out/linux_amd64/istioctl install -r 1.8 --set hub=gcr.io/istio-testing -d manifests/
Error: failed to get profile and enabled components: failed to generate IOP from profile default:
could not unmarshal merged YAML: json: cannot unmarshal number into Go value of type string

After:

$ ./out/linux_amd64/istioctl install -r 1.8.0 --set hub=gcr.io/istio-testing -d manifests/
Error: failed to get profile and enabled components: failed to read profile: validation errors (use --force to override): 
invalid revision specified: 1.8.0

[shamsher31]

/test integ-security-k8s-tests_istio

[Monkeyanator]

worth adding a test in install_test.go?