Allow to check "force sync" when hitting "Build now"

[Paul Sokolovsky]

This is offshot of lp:718478. There, it was concern that doing lazy syncs (not sync if another sync did occur recently enough, where "recently" is defined on upstream host by host basis) may lead to stale code being used for build, and thus produce not the expected results. While it is expected that such cases will be rare, there should be controls to rule them out for the selected builds (like, release one). Ideally, this control should adhere to the following requirements:

 * Affect only build in question (not the state of whole mirror for example)
 * Be applicable just to specific build run, not persist in the build config (because otherwise there will be reduced benefit of lazy syncs and increased sync/build time)

james_w proposed to add a checkbox to the frontend's build page, so user can check it before clicking "Build now" button to force using freshest upstream code (or fail if it cannot be fetched).

Formulated this way, it is also related to lp:786466.

[Paul Sokolovsky (pfalcon)]

Implementation-wise, this can be another Jenkins' build param (like we pass building config now in CONFIG param).

[Alan Bennett (akbennett)]

Due to the age of this issue, we are acknowledging that this issue will likely not be fixed. If this issue is still important, please add details and re open the issue.