Resolve installability of all dependencies

[dragonchaser]

When installing a chart that includes either optional or shared dependencies, make sure that each of them is resolvable and installable. If there is no installable candidate, cancel the whole installation and inform the user.
(This probably overlaps with #139 in some areas)

Implementation
Walk over the whole dependency tree and check if every chart/dependency is available on one of the repos.

[kkaempf]

Why should an optional dependency matter ?

[kkaempf]

imho, an optional dependency should be similar to a zypper recommends which "are silently dropped if no match is possible."

[dragonchaser]

Because it relies on the Install methods of helm and if it is installable is not determined on our side atm, meaning: if the chart is not resolvable it will fail during the installation on the helm-codes side.

[dragonchaser]

imho, an optional dependency should be similar to a zypper recommends which "are silently dropped if no match is possible."

Ah ok now I understand, still we need to resolve those on our side before hand, but of course we can drop the optional deps then.