make epoch optional in package spec (a.k.a. epoch guessing)

[MichaelMraka]

Currently when specifying packge NEVRA (both in /updates and /packages API) we require epoch to be there otherwise it's set as epoch=0. On the other hand standard output of rpm -qa does not show the epoch so common mistake could be using just NVRA instead on NEVRA which will result in empty output for packages with epoch !=0 (see #382).

Possible solution to minimize such errors could be "guessing" epoch (if not specified).
I.e. there is only a couple of packages with the same NVR and different epoch
so we can actually lookup package only by NVR and if there's a unique hit that's it (99.9%).
And if not unique let's return e.g. the one with the lowest epoch (0.1%).

[jsvob]

I just checked https://github.com/RedHatInsights/insights-core/blob/master/insights/specs/default.py#L28 and it seems Insights rules don't have access to the epoch currently either.