Lines formerly of the form '(remote,) = ctx.cluster.only(role).remotes.keys()'
and '(remote,) = ctx.cluster.only(role).remotes.iterkeys()' would fail with
ValueError and no message if there were less than 0 or more than 1 key.
Now a new function, get_single_remote_value() is called which prints out
more understandable messages.

Fixes: 7510
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Signed-off-by: Warren Usui <warren.usui@inktank.com>