Revert "do not modify existing settings if not specified in config string."

This reverts commit 1b04d1e9f3b63fb5c36d1a72b1be01926e3cef2e.
Revert "exempt capabilities from error 1006"

This reverts commit 5db6d0cdc9d516fd3bbd1ec47bdd2e6af73a6a60.
Only mandatory argument should check null, empty or undefined.

Right now, only seleniumHost is mandatory if it is provided as command
line argument.
Merge pull request #144 from bowdev/null_argument

Null argument