OCPBUGS-27965: escape '%' in proxy settings

The 'DefaultEnvironment' config from systemd accepts some % specifiers
for expansion [1]. Because of that, proxy configurations that make use
of that character need to be escaped with another '%'. However, during
proxy validation, the Installer parses the URI and fails when it
encounters a "%%" with an `invalid URL escape` error.

To mitigate the problem, we introduce a new `replace` function to the
templating system and use it to substitute "%" with "%%" causing the
following config:

```
proxy:
    httpProxy: http://user%40test:[email protected]:3128
    httpsProxy: http://user%40test:[email protected]:3128
```

to become the `10-default-env.conf`:

```
[Manager]
DefaultEnvironment=HTTP_PROXY="http://user%%40test:[email protected]:3128"
DefaultEnvironment=HTTPS_PROXY="http://user%%40test:[email protected]:3128"
```

[1] https://man7.org/linux/man-pages/man5/systemd-system.conf.5.html

Author: r4f4

data/data/bootstrap/files/etc/systemd/system.conf.d/10-default-env.conf.template

@@ -1,12 +1,12 @@
 {{if .Proxy -}}
 [Manager]
 {{if .Proxy.HTTPProxy -}}
-DefaultEnvironment=HTTP_PROXY="{{.Proxy.HTTPProxy}}"
+DefaultEnvironment=HTTP_PROXY="{{replace .Proxy.HTTPProxy "%" "%%"}}"
 {{end -}}
 {{if .Proxy.HTTPSProxy -}}
-DefaultEnvironment=HTTPS_PROXY="{{.Proxy.HTTPSProxy}}"
+DefaultEnvironment=HTTPS_PROXY="{{replace .Proxy.HTTPSProxy "%" "%%"}}"
 {{end -}}
 {{if .Proxy.NoProxy -}}
 DefaultEnvironment=NO_PROXY="{{.Proxy.NoProxy}}"
 {{end -}}
-{{end -}}
+{{end -}}

pkg/asset/ignition/bootstrap/common.go

@@ -518,6 +518,11 @@ func AddSystemdUnits(config *igntypes.Config, uri string, templateData interface
 	return nil
 }
 
+// replace is an utilitary function to do string replacement in templates.
+func replace(input, from, to string) string {
+	return strings.ReplaceAll(input, from, to)
+}
+
 // Read data from the string reader, and, if the name ends with
 // '.template', strip that extension from the name and render the
 // template.
@@ -529,7 +534,7 @@ func readFile(name string, reader io.Reader, templateData interface{}) (finalNam
 
 	if filepath.Ext(name) == ".template" {
 		name = strings.TrimSuffix(name, ".template")
-		tmpl := template.New(name)
+		tmpl := template.New(name).Funcs(template.FuncMap{"replace": replace})
 		tmpl, err := tmpl.Parse(string(data))
 		if err != nil {
 			return name, data, err