Add possibility to set command parameter as String.

[n00by]

This fix establishes the desired behavior for the "command" parameter as
described in the icinga2 docs and in the icinga2 module parameter
description:

"The command. This can either be an array of individual command arguments.
Alternatively a string can be specified in which case the shell interpreter
(usually /bin/sh) takes care of parsing the command."

Update the rspec tests.

[lbetz]

fixed in current master, thx.