If the user points S3 backups at a bucket containing other files, those
file names may not be valid configmap keys.

For example, RKE1 generates backup files with names like
`s3-c-zrjnb-rs-6hxpk_2022-05-05T12:05:15Z.zip`; the semicolons in the
timestamp portion of the name are not allowed for use in configmap keys.
Signed-off-by: Brad Davidson <brad.davidson@rancher.com>