Add feature flip for template selection

Drop 'dist' and 'group' keys from config if :template_selection
feature is not enabled.

This corresponds to worker PR
https://github.com/travis-ci/travis-worker/pull/97.
Merge pull request #390 from travis-ci/ha-feature-template-selection

Add feature flip for template selection
do not allow timeout values to equal 0, allow nil values
Add dist_group_expansion feature

When this feature is active, we expand 'dist' and 'group'
as build matrix dimension.
Merge pull request #391 from travis-ci/ha-feature-dist-group-expansion

Add dist_group_expansion feature
merge existing config when configuring requests
add a service for triggering requests via api
fix specs, reduce noise