[FABG-961] Use system cert pool when connecting to fabric-ca

[alikic]

When client.tlsCerts.systemCertPool is set to true,
TLS connections to fabric-ca instances now use system
cert pool. There is no need to have tlsCACerts.path
defined in CAs' configurations in that case.

Signed-off-by: Aleksandar Likic aleksandar.likic@securekey.com

[codecov]

Codecov Report

Merging #69 into master will increase coverage by 0.07%.
The diff coverage is 79.38%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
+ Coverage   74.61%   74.69%   +0.07%     
==========================================
  Files         185      188       +3     
  Lines       12901    12999      +98     
==========================================
+ Hits         9626     9709      +83     
- Misses       2327     2338      +11     
- Partials      948      952       +4     

Impacted Files	Coverage Δ
pkg/fab/opts.go	100.00% <ø> (ø)
pkg/msp/identityconfig.go	78.66% <68.88%> (-1.23%)	⬇️
pkg/core/mocks/mockconfigbackend.go	80.43% <86.04%> (ø)
pkg/core/config/comm/tls/certpool.go	82.60% <100.00%> (ø)
pkg/fab/endpointconfig.go	82.50% <100.00%> (ø)
pkg/msp/fabcaadapter.go	71.87% <100.00%> (+0.32%)	⬆️
pkg/msp/opts.go	90.47% <100.00%> (+0.47%)	⬆️
pkg/core/mocks/mocksigningmgr.go	0.00% <0.00%> (ø)
pkg/core/mocks/mockcorecontext.go	0.00% <0.00%> (ø)
... and 6 more

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