Honor "disabled" config flag

[tigrannajaryan]

We previously had "enabled" config flag which was not honored.
This now implements "disabled" config flag which is honored.

We decided to use "disabled" instead of "enabled" since it is
more common to have enabled items than disabled items in the config
and this reduces litter in the config file.

Github issue: #113

[pjanotti]

Please rebase and fix:

receiver/opencensusreceiver/config_test.go:62:5: unknown field 'Enabled' in struct literal of type configmodels.ReceiverSettings
receiver/opencensusreceiver/config_test.go:83:5: unknown field 'Enabled' in struct literal of type configmodels.ReceiverSettings
receiver/opencensusreceiver/factory_test.go:60:3: unknown field 'Enabled' in struct literal of type configmodels.ReceiverSettings
receiver/opencensusreceiver/factory_test.go:80:6: unknown field 'Enabled' in struct literal of type configmodels.ReceiverSettings
receiver/opencensusreceiver/factory_test.go:123:3: unknown field 'Enabled' in struct literal of type configmodels.ReceiverSettings
receiver/opencensusreceiver/factory_test.go:143:6: unknown field 'Enabled' in struct literal of type configmodels.ReceiverSettings
make: *** [vet] Error 2
The command "make travis-ci" exited with 2.

[codecov-io]

Codecov Report

Merging #181 into master will increase coverage by 0.09%.
The diff coverage is 90.24%.

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
+ Coverage   72.67%   72.77%   +0.09%     
==========================================
  Files         104      104              
  Lines        6039     6090      +51     
==========================================
+ Hits         4389     4432      +43     
- Misses       1425     1433       +8     
  Partials      225      225

Impacted Files	Coverage Δ
receiver/opencensusreceiver/factory.go	88.57% <ø> (-0.32%)	⬇️
config/configmodels/configmodels.go	0% <0%> (ø)	⬆️
config/test_helpers.go	50% <100%> (ø)	⬆️
receiver/jaegerreceiver/factory.go	95.83% <100%> (-0.17%)	⬇️
config/configv2.go	98.87% <100%> (+0.22%)	⬆️
service/service.go	68.84% <100%> (ø)	⬆️
receiver/jaegerreceiver/config.go	61.53% <80%> (+11.53%)	⬆️
config/example_factories.go	52.33% <88.88%> (-1.8%)	⬇️

---

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 6eb96e0...f8d15d1. Read the comment docs.