Fix datatype for datalist entries on Check Command generation

[LordHepipud]

Using Get-IcingaCheckCommandConfig will generate partly wrong basket configuration for datalists, as it will not set the data type property correctly.

Expected Behavior

Data type of datalists should be set to string on creation

Current Behavior

Data types are not set properly because if wrong basket variable naming

Possible Solution

Rename the variable datatype on the settings tree for datalists to data_type.

Steps to Reproduce (for bugs)

1. Export configuration with Get-IcingaCheckCommandConfig
2. Import .json into Director
3. Check datalist configuration in Icinga Director