Fix sysctl parsing in lab.conf for negative values

[tcaiazzi]

The current implementation of sysctl parsing in the lab.conf raises an exception when handling negative values.

Specifically, when a negative value is specified for a sysctl parameter, such as net.ipv6.conf.default.seg6_require_hmac=-1, the parsing mechanism fails, raising the exception MachineOptionError.

To address this issue, we need to modify the sysctl parsing logic in the lab.conf file to handle negative values appropriately.
The parsing mechanism should recognize negative values and assign them correctly to the corresponding sysctl parameter.