internal/kokoro: adds continuous.cfg
internal/kokoro: adds continuous.cfg

configure continuous builds (builds triggered on push to master)
for debugging - continuous build logs and status will not be posted
on CLs, but will only be viewable internally.

Change-Id: I72964e86895e190b3d9fc725166bde42c040e451
Reviewed-on: https://code-review.googlesource.com/10430
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go