Updates in the Filesystem v1beta2 API

[mauriciopoppe]

API Changes:

• Change all parameters from camelCase to underscores e.g. diskNumber -> disk_number
• Make sure that the comments match what the api does
• Remove the error field in the response messages
• LinkPath -> CreateSymlink, IsMountPoint -> IsSymlink

Also:

• change proto folder from v1 to v1beta2, check the setup in https://docs.google.com/document/d/1sBP8f_mwV0N_xRRQQGDwHZpU_nTHtpFxdi7LKSUtgX0/edit# and Generate v1 files from latest beta versions #120 for more details

[mauriciopoppe]

/assign