Update construct_settings overrides (googleapis#103)

* Update construct_settings overrides

Based on the discussion of googleapis#95 and googleapis/gapic-generator#70 --
this changes the interface of construct_settings a bit.

The new interface does not have the override parameters for
retrying and bundling anymore. Instead, it has a single
config_override that has the same strucure as client_config has.

API users can specify a copy of default config data with a few
modifications, but if the user-supplied config does not specify
some data, this will fall back to the default config, so
users can safely supply a part of the client config, just for
their edits.

* Style fix.

This needs to split override_config into a few functions due to
code complexity warning by PEP8.

* Update the overriding strategy.

Based on the discussion, now overriding config can override
the specified methods only, and does not merge the configurations
between the default ones and the user-provided ones.

* More merges of method configs.

* Fixes

Merge pull request googleapis#94 from geigerj/master

Change page-streaming behavior