Fix handling cli options default value

[cuonglm]

CLI options have the highest priority, so the cli options which have a
default value will always override options of other mechanisms.

To fix it, we only set default value after consolidated config process.

Close #1010

[codecov-io]

Codecov Report

Merging #1092 into master will increase coverage by 0.03%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #1092      +/-   ##
==========================================
+ Coverage   72.92%   72.95%   +0.03%     
==========================================
  Files         134      134              
  Lines        9960     9966       +6     
==========================================
+ Hits         7263     7271       +8     
+ Misses       2279     2278       -1     
+ Partials      418      417       -1

Impacted Files	Coverage Δ
cmd/config.go	76.82% <100%> (+0.79%)	⬆️
cmd/options.go	66.66% <71.42%> (+0.28%)	⬆️
core/engine.go	93.92% <0%> (+0.93%)	⬆️

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[mstoykov]

LGTM!
We should probably create an issue, with other config options which probably need this treatment, @na--
p.s. Sorry for the long delay :(

[cuonglm]

@mstoykov I did look for other flags, but sounds like only --system-tags needs this treatment.

[na--]

LGTM, thanks, and sorry for the huge delay, we had to deal with some issues surrounding k6 v0.25.0 😞

Looking at the options code, I don't immediately see any other options that would have the same issue, but I haven't actually tested them one by one... So I'd say we should merge this and deal with any others in separate PRs. @mstoykov, I think we should merge this and include it in v0.25.1, right?

[mstoykov]

This seems like low risk commit to me so I okay with that

[mstoykov]

@cuonglm I haven't looked at them, so if there are no others I am fine, and @na-- yeah we would've definitely done it in different PR except if it was ... like one other option