Allow tracer config via env vars

[yurishkuro]

Which problem is this PR solving?

• Resolves Jaeger Query expects agent at localhost #747

Short description of the changes

• invoke FromEnv on the tracer configuration object before creating a tracer so that default configuration can be overwritten via environment variables.

Tested with all-in-one by running it on different port, re-pointing the tracer to the new port, and validating messages via tcpdump:

JAEGER_AGENT_PORT=7701 go run -tags=ui ./cmd/all-in-one \
    --processor.jaeger-binary.server-host-port :7700 \
    --processor.jaeger-compact.server-host-port :7701 \
    --processor.zipkin-compact.server-host-port :7702 \
    --http-server.host-port :7703

sudo tcpdump -i any -Z root "udp port 7701" -w out.cap

[codecov]

Codecov Report

Merging #1919 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1919      +/-   ##
==========================================
- Coverage   98.48%   98.45%   -0.03%     
==========================================
  Files         198      198              
  Lines        9740     9740              
==========================================
- Hits         9592     9590       -2     
- Misses        113      114       +1     
- Partials       35       36       +1

Impacted Files	Coverage Δ
cmd/query/app/static_handler.go	86.84% <0%> (-1.76%)	⬇️

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