Add NetmaskField and update Subnet entity

Due to recent upstream builds the mask Subnet's fields are being
validated for a valid netmask value. This adds a NetmaskField that
generates valid netmasks using FauxFactory gen_netmask generator. Also
updates the Subnet's mask field to NetmaskField.

Related to #1514
Merge pull request #1515 from elyezer/update-subnet-entity

Add NetmaskField and update Subnet entity