feat: suport for multi es instances

[clarkchen]

Same Pull Request with this
#1225

Updated:

1. fix "es_port" bug
2. remove unnecessary checks
3. a comma separated string is friendly for env varibles

[nsano-rururu]

py36 run-test: commands[2] | flake8 .
./elastalert/util.py:466:1: E302 expected 2 blank lines, found 1
./elastalert/util.py:480:1: W391 blank line at end of file
./tests/util_test.py:235:63: E225 missing whitespace around operator
./tests/util_test.py:236:73: E127 continuation line over-indented for visual indent
./tests/util_test.py:239:1: E302 expected 2 blank lines, found 1
./tests/util_test.py:241:18: E231 missing whitespace after ':'
./tests/util_test.py:243:21: E222 multiple spaces after operator
./tests/util_test.py:248:35: E231 missing whitespace after ','
./tests/util_test.py:248:48: E231 missing whitespace after ','
ERROR: InvocationError for command /home/travis/build/Yelp/elastalert/.tox/py36/bin/flake8 . (exited with code 1)

[nsano-rururu]

@clarkchen

Any way you could push this PR to https://github.com/jertel/elastalert2 ?

[clarkchen]

@nsano-rururu ok, you can close this