userdata: change handling of bad userdata

Don't fail after encountering bad userdata. Continue processing the metadata
and then exit. This will allow people with bad userdata to actually log in and
see the error.
Merge pull request #225 from crawford/exit

userdata: change handling of bad userdata
flags: add oem flag

The oem flag will allow each of the OEMs to specify one flag only, acting as a
shortcut to their specific configuration. This will allow us to update which
options each OEM uses when running cloudinit.
flags: move the flags into their own namespace