Improve specifying Metalware dependencies

[bobwhitelock]

Currently specifying a new package required by Metalware requires that it should be:

• added to bin/setup (for installation using yum in development);

• added to scripts/os/el7.sh (for installation using yum in production);

• added to bin/install-ci-dependencies (for installation using apt-get in CI);

and there's also scripts/os/el6.sh for installing in production on EL6, which is not up-to-date (or used?).

This gives lots of scope for things getting inconsistent or out of sync; we should improve this.

[WilliamMcCumstie]

This issue can be addressed the next time a new dependency is added.

[bobwhitelock]

The same issue applies to https://github.com/alces-software/underware.