support different deployment tasks

[trz42]

Extends the upload script to support different deployment tasks such as adding, deleting and updating contents of a CernVM-FS repository. For now, the script is called by the bot with default parameters corresponding to adding a tarball. This can then be used by the automated ingestion procedure on the Stratum-0 for processing different types of tasks.

In a subsequent PR, we will add support for different tasks created by job scripts. These could add files named

_bot_jobJOBID.task.SEQUENCE

The files would simply contain information such as

action = 'add' or 'delete' or 'update'
payload = /path/to/payload
order = some_32bit_number_used_for_ordering
mode = 'order' or 'timestamp'

The bot would do one upload for each of these task files.