This PR makes it possible to modify the koding user,
that is running the Klient service on a remote vm.

In particular the following user_data will currently
make the template parsing to fail:

    user_data: |
      #cloud-config
      users:
      - name: ${var.koding_account_profile_nickname}
        groups:
        - docker

This PR will merge the koding user object with the
default one, defined by kloud. In effect, the resulting
user will be added to a docker group as well.

This technique can be also used e.g. to add an ssh key
for the koding user, like:

    user_data: |
      #cloud-config
      users:
      - name: ${var.koding_account_profile_nickname}
        ssh-authorized-keys:
        - ssh-rsa AAAAB3NzaC1yc2EA... koding-350298856

A "userdata" is a JSONPath which allows for locating
a user_data (cloud-init) key within a stack template tree.

Locating cloud-init in the tree is going to allow for
provider-agnostic manipulation of stack template content,
like creating new and custom templates on-the-fly (as oppose
to dummy templates received from JTemplate.Sample).

The templates are meant to be manipulated client-side (e.g. kd),
before sending them over to koding.

Allows for disabling metrics with:

    $ kd config set noMetrics true

Works around #11201.