Replace s3 URIs with signed URLs for metal.server when creating the bootscript

Replace s3:// urls with signed urls for metal.server (#51)

When creating the bootscript bss will now convert s3:// urls to signed
urls. This conversion is done for the params field for the metal.server field.

For example, it will convert the following to a signed url:
'metal.server=s3://bucket/path'

It will leave other s3 uris alone. For example this will remain untouched:
'root=craycps-s3:s3://boot-images/2b63caf8'

Jira: CASMHMS-5656