Add test coverage to opts and refactor

- Refactor opts.ValidatePath and add an opts.ValidateDevice
  ValidePath will now accept : containerPath:mode, hostPath:containerPath:mode
  and hostPath:containerPath.
  ValidateDevice will have the same behavior as current.

- Refactor opts.ValidateEnv, opts.ParseEnvFile
  Environment variables will now be validated with the following
  definition :
  > Environment variables set by the user must have a name consisting
  > solely of alphabetics, numerics, and underscores - the first of
  > which must not be numeric.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>