Runbot custom

Runbot with custom build instructions.
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This is useful for custom installation and deployment scripts such as buildout.

Add option in repo form view for custom builds. When checked:
* Modules to test becomes required attribute.
* Custom build directory can be specified. This is where the repo will be cloned in the build directory.
* Custom server path is required to be specified. This is tells runbot which executable script to run.
* Custom server flags can be specified to add to execution. E.g. --workers=0
* Pre-build commands can be run such as additional fetch scripts or buildout.