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
this, https://github.com/rabbitmq/rabbitmq-server/blob/v1_7/docs/rabbitmqctl.1.pod#output-escaping makes for a slightly more readable URL. (it's a pity that doc isn't up at http://www.rabbitmq.com/previous.html )