Make expand work by default

[andrewmains12]

What this PR does / why we need it:

My previous config PR didn't have the default behavior I expected it to--go.uber.org/config only expands env vars if you pass it the option.

I made x/config pass os.LookupEnv as an option to the config library by default.
Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

${VAR_NAME} strings will now actually be expanded.

Does this PR require updating code package or user-facing documentation?:

NONE

[codecov]

Codecov Report

Merging #2033 into master will decrease coverage by 5.8%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #2033     +/-   ##
========================================
- Coverage    72.5%   66.7%   -5.9%     
========================================
  Files         999     995      -4     
  Lines       85840   86121    +281     
========================================
- Hits        62315   57522   -4793     
- Misses      19295   24407   +5112     
+ Partials     4230    4192     -38

Flag	Coverage Δ
#aggregator	82.5% <ø> (+14.2%)	⬆️
#cluster	65.4% <ø> (+1.7%)	⬆️
#collector	67.1% <ø> (+27.9%)	⬆️
#dbnode	70.9% <ø> (+10%)	⬆️
#m3em	46.8% <ø> (+14%)	⬆️
#m3ninx	74.2% <ø> (+50.9%)	⬆️
#m3nsch	51.1% <ø> (+5%)	⬆️
#metrics	17.7% <ø> (-5.4%)	⬇️
#msg	57.8% <100%> (-15.3%)	⬇️
#query	47.6% <ø> (-25%)	⬇️
#x	55.3% <100%> (+37.3%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ee24d35...cfca092. Read the comment docs.

[codecov]

Codecov Report

Merging #2033 into master will decrease coverage by 5.8%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #2033     +/-   ##
========================================
- Coverage    72.5%   66.7%   -5.9%     
========================================
  Files         999     995      -4     
  Lines       85840   86121    +281     
========================================
- Hits        62315   57522   -4793     
- Misses      19295   24407   +5112     
+ Partials     4230    4192     -38

Flag	Coverage Δ
#aggregator	82.5% <ø> (+14.2%)	⬆️
#cluster	65.4% <ø> (+1.7%)	⬆️
#collector	67.1% <ø> (+27.9%)	⬆️
#dbnode	70.9% <ø> (+10%)	⬆️
#m3em	46.8% <ø> (+14%)	⬆️
#m3ninx	74.2% <ø> (+50.9%)	⬆️
#m3nsch	51.1% <ø> (+5%)	⬆️
#metrics	17.7% <ø> (-5.4%)	⬇️
#msg	57.8% <100%> (-15.3%)	⬇️
#query	47.6% <ø> (-25%)	⬇️
#x	55.3% <100%> (+37.3%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ee24d35...cfca092. Read the comment docs.