19980: System properties only require arv: prefix

19980: System properties only require arv: prefix

The decision to require a following alphabetic character was
well-intentioned but misplaced: it doesn't stop these properties from
existing, just changes the layer they would be defined at.

It might make sense to add some name rules to properties, but that would
be better done across the board as a separate story, not as a specific
corner case of system properties.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>