add warning or Caveat for existing Config directory

[dziggas]

When installing from this tap, it would be convenient to issue a warning or display a Caveat to verify that no existing config directory conflicts exist.

Assuming a clean install from from a non-elastic tap (currently this installs 6x):
brew install elasticsearch

And then a subsequent uninstall (perhaps because 7x was desired)
brew uninstall elasticsearch

The config directory is left behind at /usr/local/etc/elasticsearch/

Then, when running
brew install elastic/tap/elasticsearch-full

The following will occur:

➜  ~ elasticsearch
Exception in thread "main" java.lang.RuntimeException: starting java failed with [1]
output:
[0.003s][error][logging] Error opening log file 'logs/gc.log': No such file or directory
[0.003s][error][logging] Initialization of output 'file=logs/gc.log' using options 'filecount=32,filesize=64m' failed.
error:
OpenJDK 64-Bit Server VM warning: Option UseConcMarkSweepGC was deprecated in version 9.0 and will likely be removed in a future release.
Invalid -Xlog option '-Xlog:gc*,gc+age=trace,safepoint:file=logs/gc.log:utctime,pid,tags:filecount=32,filesize=64m', see error log for details.
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.
    at org.elasticsearch.tools.launchers.JvmErgonomics.flagsFinal(JvmErgonomics.java:118)
    at org.elasticsearch.tools.launchers.JvmErgonomics.finalJvmOptions(JvmErgonomics.java:86)
    at org.elasticsearch.tools.launchers.JvmErgonomics.choose(JvmErgonomics.java:59)
    at org.elasticsearch.tools.launchers.JvmOptionsParser.main(JvmOptionsParser.java:92)

Java:

➜  ~ java -version
openjdk version "1.8.0_232"
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_232-b09)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.232-b09, mixed mode)

Not a big issue - clearing out the dirs mentioned in the Caveats clears it up - but a sanity check in the formula might save somebody some time down the road.