tag configMap data nodes as 'string'

[natasha41575]

Fixes #3193

First commit moves some code around and adds a (broken) test demonstrating current behavior. Second commit provides the fix for #3193.

When underlying go-yaml sees non-string values (e.g. integers or booleans), they must be explicitly tagged as strings in order to be quoted and processed correctly by other yaml-processing code.

With the following input:

data := map[string]string{
    "normal string": "abc",
    "integer":       "8081",
    "float":         "1.23",
    "bool":          "true",
}
m, err := NewConfigMap(data)
fmt.Println(m.MustString())

Before this fix:

apiVersion: v1
kind: ConfigMap
metadata:
  name: function-input
data: {normal string: abc, integer: 8081, float: 1.23, bool: true} # this is invalid

After this fix:

apiVersion: v1
kind: ConfigMap
metadata:
  name: function-input
data: {normal string: abc, integer: "8081", float: "1.23", bool: "true"}

[natasha41575]

@droot PTAL