Description of problem:
katello-installer with --capsule-dhcp-range is not able to set to default when using "false" or it doesn't check the syntax.

Version-Release number of selected component (if applicable):
6.1.3

How reproducible:
install satellite witch capsule and dhcp support

Steps to Reproduce:
1. After the satellite with capsule is installed, run:
2. katello-installer --capsule-dhcp-range "1 1" -> will fail, in the config
   will be row like this: range "1 1"
3. katello-installer --capusle-dhcp-range false  -> will fail, in the config
   will be row like this: range false

Actual results:
  There is wrong value for dhcp range

Expected results:
  User receive an error message, when the syntax is not correct or in the case of "false", which supposed to be default, the range will be set to default