[LIVY-560] Added support for hostnames that resolve to an external address

[smrtl]

What changes were proposed in this pull request?

In RSC context when the local hostname does not resolves to a loopback address, the canonical hostname is used and sent to other nodes.
This is a problem when running Livy on Kubernetes as another node won't be able to resolve the hostname. The proposed change is to return the IP address of the host when it's hostname resolves to an external IP.

Closes https://issues.apache.org/jira/browse/LIVY-560
Probably closes https://issues.apache.org/jira/browse/LIVY-372 too

How was this patch tested?

This was tested by successfully running Livy with Spark 2.4.0 on Kubernetes 1.13.2

[codecov-io]

Codecov Report

Merging #145 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #145      +/-   ##
============================================
- Coverage     68.59%   68.54%   -0.05%     
  Complexity      904      904              
============================================
  Files           100      100              
  Lines          5662     5666       +4     
  Branches        848      848              
============================================
  Hits           3884     3884              
- Misses         1225     1230       +5     
+ Partials        553      552       -1

Impacted Files	Coverage Δ	Complexity Δ
rsc/src/main/java/org/apache/livy/rsc/RSCConf.java	82.72% <0%> (-3.13%)	7 <0> (ø)
...c/main/scala/org/apache/livy/repl/ReplDriver.scala	30.76% <0%> (-2.57%)	7% <0%> (ø)
...c/src/main/java/org/apache/livy/rsc/RSCClient.java	73.91% <0%> (-1.25%)	20% <0%> (ø)
...main/scala/org/apache/livy/server/LivyServer.scala	35.96% <0%> (+0.49%)	11% <0%> (ø)	⬇️
...ain/java/org/apache/livy/rsc/driver/RSCDriver.java	78.81% <0%> (+0.84%)	41% <0%> (ø)	⬇️

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[smrtl]

Closing this since it is covered by the great work done by @jahstreet, cf. #167, #249, #252