Alloc exec only works when task is passed as a flag and not an arg.
Alloc logs currently accepts either, but alloc signal and restart only
accept task as an arg. This adds -task as a flag to the other alloc
commands to make the cli UX consistent. If task is passed as a flag and
an arg, it ignores the arg.
The `nomad alloc logs` command does not remove terminal escape sequences for
color from the log outputs of a task. Clarify that the standard `-no-color`
flag, which does apply to Nomad's error responses from `nomad alloc logs`,
does not apply to the log output.
