[LIVY-715][DOC] The configuration in the template is inconsistent with LivyConf.scala

[captainzmc]

What changes were proposed in this pull request?

When I test livy impersonation found that, in livy.conf.template the value of livy.impersonation.enabled is true. So I thought impersonation was enabled by default.             
However, impersonation was not turned on when we test. I found that the real configuration in LivyConf. scala is false. 
This can mislead users.

How was this patch tested?

no need

[captainzmc]

@jerryshao Just change the doc to avoid misleading the user. Could please help review it？

[codecov-io]

Codecov Report

Merging #261 into master will increase coverage by 0.34%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #261      +/-   ##
============================================
+ Coverage     67.85%   68.19%   +0.34%     
- Complexity      937      941       +4     
============================================
  Files           102      102              
  Lines          5876     5876              
  Branches        891      891              
============================================
+ Hits           3987     4007      +20     
+ Misses         1315     1299      -16     
+ Partials        574      570       -4

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/org/apache/livy/rsc/driver/RSCDriver.java	80.75% <0%> (+2.09%)	45% <0%> (+2%)	⬆️
...cala/org/apache/livy/scalaapi/ScalaJobHandle.scala	55.88% <0%> (+2.94%)	7% <0%> (ø)	⬇️
rsc/src/main/java/org/apache/livy/rsc/rpc/Rpc.java	82.24% <0%> (+2.95%)	12% <0%> (ø)	⬇️
...in/java/org/apache/livy/rsc/rpc/RpcDispatcher.java	62.24% <0%> (+3.06%)	20% <0%> (+1%)	⬆️
...c/src/main/java/org/apache/livy/rsc/RSCClient.java	77.1% <0%> (+3.61%)	20% <0%> (ø)	⬇️

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[yiheng]

Yes. This template configuration is conflicted with this code
https://github.com/apache/incubator-livy/blob/master/server/src/main/scala/org/apache/livy/LivyConf.scala#L72

+1

[jerryshao]

LGTM.