MB-7604:cbcollect_info: curl /diag passing credentials properly

I.e. by passing them as separate argument and avoiding shell argument
expansion altogether.

As part of doing it I've made sure Task supports list
commands. I.e. command lines with explicit arguments which doesn't
need any shell expansion.

Change-Id: Ibaf16430306a2c6c5c5dfb614ad95f7cdf944244
Reviewed-on: http://review.couchbase.org/25428
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>