Previously if the user provided a non-conforming string, it would blow up with
`java.lang.StringIndexOutOfBoundsException: String index out of range: -1`
which is not a *helpful* error message.
Also updated the documentation to make the possible setting values more clear.
Close#5752
`allocation.disable_new_allocation`, `allocation.disable_allocation`, `allocation.disable_replica_allocation`,
in favour for the enable allocation decider which has a single option `allocation.enable` wich can be set to the following values:
`none`, `new_primaries`, `primaries` and `all` (default).
Closes#4488
Matthew L Daniel
Martijn van Groningen
Clinton Gormley
Simon Willnauer
Lee Hinman