optimize installation of gadgets when there are more than one gadgets

[brant-ruan]

E.g. for cve-2020-15257 which needs two gadgets (docker-ce and containerd), if Metarget finds docker-ce with specified version has been installed, it should just pop this gadget out of the temp_gadgets list, so that only containerd need to be installed later. Currently, the logic is, if not all the gadgets have been installed, Metarget will install all of them once again, which is not elegant :P