Add resolve_template function to easyconfig instances

[Flamefire]

(created using eb --new-pr)

Not only does this reduce the (non-trivial) code duplication, it also allows e.g. easyblocks to resolve a template without setting an EC parameter. e.g. self.cfg.resolve_template('lib/%(pyshortver)s') might be useful.

[boegel]

@Flamefire Can you look into adding a dedicated test for the resolve_template method?

[Flamefire]

@Flamefire Can you look into adding a dedicated test for the resolve_template method?

I though it was not necessary: The tests for checking the template values already cover this via the getters and a test if ec.resolve_template(x) == resolve_template(x, ec.template_values) doesn't seem to provide much value as that is basically the implementation of the method.

So only thing which I'd say is worth checking, is whether the template dict gets initialized first time this gets called to detect easier if it gets removed by some later change, although the other tests would fail already.
But I'm not sure how to "legally" create an easyconfig instance without the template_values being initialized.

[Flamefire]

I added a test and tried to trigger the case where template_values is not set. Turns out this is impossible, so I removed the condition.