Add `gscloud postgres releases`

[nvthongswansea]

Similar to #113. Since we are adding postgres resource to gridscale tf provider, it is convenient for the user to use gscloud to get the releases of postgresSQL service in gridscale.
Example output:

$ gscloud postgres releases 
10
11
12
13   

[bkircher]

Hmm, let's make this gscloud postgresql releases. Think that the "official" name might be better everywhere, right?

[fkr]

Basically the various sql databases are - from a product view - flavours of the "gridsql". Since we use that terminology everywhere, it might make sense to have it under 'gridsql' in gscloud as well.

[bkircher]

Good point. Still, the commands would then look like

$ gscloud gridsql postgresql releases
$ gscloud gridsql mariadb releases

vs.

$ gscloud postgresql releases
$ gscloud mariadb releases

As well as the already existing

$ gscloud kubernetes releases

Personally I would go with the second option because of typing ergonomics 😉 . What do you guys prefer?

[MarcHarriss]

Second one imo :)

[itakouna]

I would also prefer gscloud <product_name> < flavour>. This keeps gscloud consistent with the API and Panels.

[bkircher]

Second one +1 here, too…. so would go with that