New command to check task host account cylc versions.

Addresses #853

```
% cylc check-versions --help
Usage: cylc [discovery] check-versions [OPTIONS] SUITE 

Check the version of cylc invoked on each of SUITE's task host accounts
when CYLC_VERSION is set to 5.4.6-46-gd154 (i.e. *this* version).
Different versions are reported but are not considered an error unless
the -e|--error option is specified, because different cylc versions are
not necessarily (nor usually) incompatible.

It is recommended that cylc versions be installed in parallel and access
configured via the cylc version wrapper as described in the cylc INSTALL
file and User Guide. Users then get the latest installed version by
default, or (like tasks) a particular version if $CYLC_VERSION is defined.

Remote cylc versions are interrogated like this:
  ssh user@host \
     "bash --login -c 'CYLC_VERSION=5.4.6-46-gd154 cylc -v'"
A login shell is used because task job scripts currently source login 
scripts explicitly at start-up to configure access to cylc.

Arguments:
   SUITE               Suite name or path

Options:
  -h, --help            show this help message and exit
  -e, --error           Exit with error status if 5.4.6-46-gd154 is not
                        available on all remote accounts.
...
```