Add allow_empty_roles to control aborting on roles with no hosts.

[mdkent]

Add allow_empty_roles to control aborting on roles with no hosts.

This added flexibility allows you to define base roles that might not necessarily exist in each deploy destination.

eg:

config/deploy.yml

servers:
  web:
    cmd: bin/web
    <other important common config>

  jobs:
    cmd: bin/jobs
    <other important common config>

config/deploy.beta.yml

servers:
  web:
    cmd: bin/web
    hosts:
    - 1.1.1.1
    - 1.1.1.2

# No jobs on betas!

config/deploy.production.yml

servers:
  web:
    hosts:
    - 1.1.1.3
    - 1.1.1.4

  jobs:
    hosts:
    - 1.1.1.5
    - 1.1.1.6

config/deploy.staging.yml

servers:
  web:
    hosts:
    - 1.1.1.7
    - 1.1.1.8

  jobs:
    hosts:
    - 1.1.1.9
    - 1.1.1.10

without this a kamal deploy -d beta would fail with No servers specified for the jobs role.