Move all ports commands under the gp CLI namespace

[loujaybee]

Following the introduction of gp ports list, we now have two patterns for the CLI, top-level commands, and nested commands. Nested commands is the pattern followed for gp tasks, for instance, but ports commands are still at the top level.

I suggest adding an alias from:

gp forward-port to gp ports forward (this should be renamed)
gp await-port to gp ports await

And removing references to the original command help text in all docs.

[akosyakov]

Do we still need gp ports forward at all?