[CMSP-217] Accept siteID key as either int or string

[pwtyler]

As part of implementing pantheon-systems/terminus-yml-validator-plugin#1, I discovered it is difficult, if not impossible, for PHP to differentiate between siteID keys being an int, or a string that could be cast to an int.

Rather than jumping through a lot of hoops on the PHP side, I opted to make the go validator a bit more flexible.

TODO:

• assert how the search-replace script behaves if the key is an int in one case and a string in the other, and adapt accordingly (this can be done as part of upgrading sites-yml-validator in the job worker and does not need to block PR).

[pwtyler]

• TODO: Update Readme.