Clarify tmt mission and compare with others

[lukaszachy]

Basically follow up to #1218 (reply in thread)

I think we feel what tmt should be about but it is hard to put it down.
So we know what is in the scope, what is just a "good will" to make life easier, who do we "compete" with and why we win/lose.

IMHO Miro's "imagine running travis or gitlab-ci yaml from your localhost" captures most of it, but there are bits tmt is better or worse with and we should have them written down.