Strip backslashes from the output of rabbitmqctl

The output of `rabbitmqctl(1)` escapes backslashes. This was documented
in the `rabbitmqctl` man page[1] but was removed when the manpage was
converted to DocBook XML[2].

This change causes extraneous backslashes to be stripped to prevent
this module from misreading escaped characters in regular expressions
and repeatedly applying the same change.

[1]:
https://github.com/rabbitmq/rabbitmq-server/blob/16418a9488e15c4d8ef3bfa9fce69190fb8ec796/docs/rabbitmqctl.1.pod#output-escaping
[2]:
https://github.com/rabbitmq/rabbitmq-server/commit/60c14baa96118e587b12716b49f7d66e487a5939#diff-b96d4e7ccd3d984656b8ab3534e6b460
Use shorter URL for RabbitMQ documentation

As suggested in
https://github.com/puppetlabs/puppetlabs-rabbitmq/pull/225.
Remove trailing whitespace
Merge pull request #225 from alphagov/strip-backslashes

Strip backslashes from the output of rabbitmqctl