Added get-dependencies option to install_pack.py

[jk464]

Enhances install_pack.py so that you can pass an optional flag to it (--get-dependencies) that will for a given pack when it is installed will also install any other packs the parent pack depdends on it.

Useful when using https://github.com/StackStorm/st2packs-dockerfiles/ to build a st2packs image, the user only has to provide the list of packs they want, without having to determine if they have any further packs as depdencies.

[guzzijones]

needs a changelog entry

[jk464]

@guzzijones I've added a Changelog entry now