cp rework 2

[abiswal2001]

Couple notes fixing discrepancies between cp and how it works in linux:

• cp will overwrite an existing file with new file contents if the dest file prefix already exists in the dest bucket, no more name concatenation (fixes Skylark cp concatenating object names when using gateways #280)
• trailing slashes are required for directories, as is convention with s3 and in linux convention
• trailing slashes are optional at root bucket level
• "-r" flag was not added as by default directories are recursively copied.

[parasj]

LGTM!