debile-remote: add commande update-builder

[clemux]

debile-remote should be able to modifiy a builder's configuration

The more urgent functionality is the ability to change a builder ip address (for ip-based authentication).

Relevant files are:

• https://github.com/opencollab/debile/blob/master/debile/utils/cli.py#L51
• https://github.com/opencollab/debile/blob/master/debile/master/interface.py#L181

This diff can give some pointers: 0fdea93

Basically:

1. add a "update_builder" handler in the server interface (debile/master/interface.py)
2. add a "update-builder" command to debile-remote (debile/utils/cli.py)