RFE: Add feature for listing relations together with charm options

Examples:

Instead of:

applications:
  mysql:
    charm: cs:percona-cluster
    bindings:
      "": *oam-space
      cluster: *internal-space
      shared-db: *internal-space
      ha: *internal-space
      db: *internal-space
      db-admin: *internal-space
    options:
      innodb-buffer-pool-size: 256M
      enable-binlogs: True
      performance-schema: True
      max-connections: *mysql-connections
      tuning-level: *mysql-tuning-level
      source: *openstack-origin
    num_units: 1
    to:
    - lxd:0
  keystone:
    charm: cs:keystone
    bindings:
      "": *oam-space
      public: *public-space
      admin: *admin-space
      internal: *internal-space
      shared-db: *internal-space
    options:
      vip: *keystone-vip
      admin-password: openstack
      openstack-origin: *openstack-origin
      preferred-api-version: 3
      region: *openstack-region
      token-provider: 'fernet'
      worker-multiplier: *worker-multiplier
    num_units: 3
    to:
    - lxd:6
    - lxd:7
    - lxd:8
relations:
- [ "keystone:shared-db", "mysql:shared-db" ]

You could use something like:

applications:
  mysql:
    charm: cs:percona-cluster
    bindings:
      "": *oam-space
      cluster: *internal-space
      shared-db: *internal-space
      ha: *internal-space
      db: *internal-space
      db-admin: *internal-space
    options:
      innodb-buffer-pool-size: 256M
      enable-binlogs: True
      performance-schema: True
      max-connections: *mysql-connections
      tuning-level: *mysql-tuning-level
      source: *openstack-origin
    num_units: 1
    to:
    - lxd:0
  keystone:
    charm: cs:keystone
    bindings:
      "": *oam-space
      public: *public-space
      admin: *admin-space
      internal: *internal-space
      shared-db: *internal-space
    options:
      vip: *keystone-vip
      admin-password: openstack
      openstack-origin: *openstack-origin
      preferred-api-version: 3
      region: *openstack-region
      token-provider: 'fernet'
      worker-multiplier: *worker-multiplier
    num_units: 3
    to:
    - lxd:6
    - lxd:7
    - lxd:8
    relations:
      - shared-db/mysql:shared-db

This may seem like a small change but would greatly simplify copying parts of bundles around.

Totally agreed. Some comments: defining a relation on at least one of the charms should be enough to get the relation set; if the relations differs (e.g. charm 1 I define endpoint A and charm 2 I define endpoint B), it fails

I like this very much.

Seems like this could be implemented relatively easily. With a first pass parsing of the bundle, any per-application relation could be simply translated to a relation in the relations section.

If we see:

  applications:
    oneapp:
      relations:
        - interfaceX/otherapp:interfaceY

It would get directly translated to:

  relations:
    - ["oneapp:interfaceX", "otherapp:interfaceY"]

Any validation or further parsing can continue as is.

Of course, this wouldn't change bundle exporting. That would be a whole other level.