Merge lp://staging/~adeuring/charmworld/1206659-simpler-es-mapping into lp://staging/~juju-jitsu/charmworld/trunk

Define a (mostly) static mapping for charms.

This branch is a first step to fix bug 1206659: Charmworld indexes far too
many fields due to Elasticsearch dynamic mapping.

Limitations:

  - It defines a static mapping only for charms, not for bundles
  - The attributes "requires" and "provides" of a charm still have
    a dynamic mapping.

Details:

The method ElasticSearchClient.put_mapping() now defines a static mapping
for charms. All fields needed for text searching, for filtering and ordering
are indexed, but no other fields.

charm['requires'] and charm['provides'] are dictionaries with user-defined
keys, hence it is impossible to define a static mapping. Fixing bug
1194907 will allow to define a static mapping for these fields too.

test_mapping_changes_during_indexing() shows that indexing a charm
changes only the mapping for requires/provides.

TestCharmSource.test_save_deletes_on_error() needed a change: Attempts
to index two charm with different types of a property named "b" now just
work because charm['b'] is now ignored by ES.

Similary, TestCharmSource.test_sync_index_script() needed a change
because the field "maintainer" is no longer indexed. (When I tried to
fix this test, I noticed bug 1214328.)

Testing the new migration showed that ES considers the new static mapping
to be compatible with the old dynamic mapping: If
ElasticSearchClient.put_mapping() is called when the index already contains
data with the previous dynamic mapping, the new index specifications are
simply accepted but the mapping stays dynamic. Hence I added the parameter
force_reindex to charmworld.search.update().