Update tests to run with multiple elasticsearch versions

[popravich]

• Enabled tags: 1.7, 2.4, 5.2.
• Fixed most 5.x compatibility issues;
• Add stored_fields in mget, search, explain;
• Add wait_for_no_relocating_shards parameter in health;
• Add filter, token_filter, char_filter params in analyze;
• Add force_merge (renamed optimize);
• Add ignore_idle_threads param in hot_threads;
• Some small fixes;

[codecov-io]

Codecov Report

Merging #123 into master will increase coverage by 1.49%.
The diff coverage is 95.88%.

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
+ Coverage   94.98%   96.47%   +1.49%     
==========================================
  Files          10       10              
  Lines        1256     1333      +77     
  Branches       15       48      +33     
==========================================
+ Hits         1193     1286      +93     
+ Misses         56       37      -19     
- Partials        7       10       +3

Impacted Files	Coverage Δ
tests/test_cluster.py	100% <100%> (ø)	⬆️
tests/test_indices.py	97.4% <100%> (+2.05%)	⬆️
tests/test_cat.py	100% <100%> (ø)	⬆️
tests/test_pool.py	99.25% <100%> (ø)	⬆️
tests/test_snapshot.py	72.6% <66.66%> (-0.38%)	⬇️
tests/test_nodes.py	90.9% <83.33%> (-9.1%)	⬇️
tests/conftest.py	86.81% <87.17%> (+1.09%)	⬆️
tests/test_client.py	98.98% <97.89%> (+4.51%)	⬆️

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[popravich]

Related to #48, #72, #112.
Fixes #73