Updated App model validation logic

Marathon UI doesn't allow scale and suspend operations, if cmd string is empty.
Cmd string can be empty if an executor and container.image are provided. Today,
executor and container.image cannot be provided through UI, but v2 API supports it.

This commit introduces an additional check, that verifies if executor and
container.image are provided, if cmd string is empty.

Updated image check logic, to only check for docker instead of docker:///

Updated gitignore to ignore eclipse specific files

Removed required attribute for cmd field
Merge branch 'master' of https://github.com/mohitsoni/marathon