roomservice: Change default remote to omnirom

Using the default of github results in people unable to use gerrit
to submit patches to repos brought in by roomservice

Change-Id: Idccd6c35bf81a35135e893ed03f6c5745059ec82
roomservice: Fix handling of devices with _ in their name

Currently, roomservice takes every _ in the device tree name and
replaces it with /

Instead, strip the device name (such as pollux_windy) from the repo
name before replacing _ with / and then append it again when we're done

Change-Id: Ieabe85ecd8829c959a7296e5cd73f577879c4ffc