Ensure CMEW can be run at other sites

Following on from #38, the changes made to address the point:

- Include the `opt/rose-suite-metoffice.conf` without having to use the `-O metoffice` command line option.

should not prevent someone from running CMEW at another site, e.g. JASMIN.

Document all this!

**Acceptance criteria (default)**

- **Given** access to CMEW and any site
- **When** the command `cylc vip` is run
- **Then** the workflow errors and a helpful message returned, i.e. "use -O when running, see metadata for available sites"

**Acceptance criteria (Met Office)**

- **Given** access to CMEW and the Met Office
- **When** the command `cylc vip -O metoffice` is run
- **Then** CMEW successfully runs at the Met Office

**Acceptance criteria (JASMIN)**

- **Given** access to CMEW and JASMIN
- **When** the command `cylc vip -O jasmin` is run
- **Then** CMEW successfully runs at JASMIN

**Details**

- There should be no defaults, i.e. `cylc vip` should error with a helpful message. The message should say something like "use -O  when running, see metadata for available sites".
- Setting either `-O metoffice` on the command line or updating `SITE` in the `rose-suite.conf` file (either manually or via `rose edit`) should produce the same outcome.
- Estimate: < 1 day

Start with opt/rose-suite-metoffice.conf and rose-suite.conf, then trace how cylc vip and the SITE setting select a site. Check the existing Met Office configuration and metadata for available site options. Done means the default command gives the requested guidance, while -O metoffice and -O jasmin work and match the SITE setting.

Written by the indexing model from the issue text.