Bad call to configure_sources corrupts sources.list and triggers fail loop.

[Stuart Bishop]

If you call fetch.configure_sources with an invalid 'deb ...' line, it is put into sources.list without validation. This throws 'apt-get update' into an infinite loop.

2014-12-01 15:47:20 INFO install E: Malformed line 60 in source list /etc/apt/sources.list (dist parse)
2014-12-01 15:47:20 INFO install E: The list of sources could not be read.
2014-12-01 15:47:20 INFO juju-log Couldn't acquire DPKG lock. Will retry in 10 seconds.
2014-12-01 15:47:30 INFO install E: Malformed line 60 in source list /etc/apt/sources.list (dist parse)
2014-12-01 15:47:30 INFO install E: The list of sources could not be read.
2014-12-01 15:47:30 INFO juju-log Couldn't acquire DPKG lock. Will retry in 10 seconds.