When deploying replicas, Coriolis users can specify scripts that will be executed
during os-morphing, right after the OS partition is mounted on the minion instance.

In some situations, this is too late since user scripts may be needed in order
to be able to mount the OS disk, for example if it’s encrypted.

In other situations it’s too early. Some scripts may need to be executed on
the replica instance. This may require provider assistance.

To accommodate these use cases, we’ve extended the deployment API, allowing the
user to specify when a given script should be executed. Each script may specify
one of the following execution phases:

* osmorphing-pre-os-mount
* osmorphing-post-os-mount (default)
* replica-initial-boot (TBD)

Samples:

* Explicit phase
    --user-script-global linux=/some/script.sh,phase=osmorphing-pre-os-mount
* Implicit phase, defaults to osmorphing-post-os-mount
    --user-script-global linux=/some/script.sh
* Repeating the flag, specifying multiple scripts:
    --user-script-instance some-instance=/some/script.sh,phase=osmorphing-pre-os-mount
    --user-script-instance some-instance=/other/script.sh,phase=osmorphing-pre-os-mount
    --user-script-instance some-instance=/another/script.sh,phase=osmorphing-post-os-mount

We'll pass type=comma_separated_kv_to_dict when defining
user script args, letting it do the parsing for us.

* specify the supported phases and which one is the default
* mention the fact that when updating transfers, the script path
  can be omitted in order to unregister it