[rails_deploy_ssh_keypair_copy] Allow users to disable SSH keypair copies

Added new rails_deploy_ssh_keypair_copy parameter
[bug/use-rails_deploy_git_version-variable]

Fixed a bug where the rails_deploy_git_version variable wasn't being
used correctly
[remove-postgres-dependency]

When using the role on a MYSQL app, the postgres library was
still automatically installed. ImageMagick was also similarly
needlessly installed.

I've thus moved the dependencies to a variable, and included an
example for postgres and imagemagick.
Merge branch 'make-ssh-keypair-copy-optional'
Merge branch 'remove-postgres-dependency'