Change the format of public key configuration according to TIP-29

[muXxer]

According to TIP-29, the format in the config file for milestone public keys should be changed, because its easier to read and we also can change the threshold of applicable keys per range.

• Index Range providing the interval of milestone indices for which this entry is valid. The interval must not overlap with any other entry.
• Applicable Public Keys defining the set of valid public keys.
• Signature Threshold specifying the minimum number of valid signatures. Must be at least one and not greater than the number of Applicable Public Keys.