crbuild: do not require lease duration
crbuild: do not require lease duration

Made lease_expiration_ts parameter in buildbucket.lease optional. It is
easier to play in API explorer this way.

R=vadimsh@chromium.org, sergeyberezin@chromium.org
BUG=

Review URL: https://codereview.chromium.org/815063002

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra