Possible improvements to dhcp-queue-control member parsing

The following discussion from !120 (merged) should be addressed:

• @fdupont started a discussion: (+1 comment)

  This can be postponed but the dhcp-queue-syntax is not defined enough:

  • you should create a context for the two keywords
  • to make it easy to extend just add a STRING COLON value rule (value is the generic JSON value defined at the beginning of dhcpX_parser.yy and is used for instance to define a generic mao, cf map_value rule)

@tmark

The basic rules for dhcp-queue-control are:

1. Require enable-queue
2. Ensure that if queue-type is specified, it is a string (it can have any arbitrary value)
3. beyond those two rules, it can contain arbitrary content

I wasn't sure how to go about accomplishing rule 3, and do not want to use user-context. The parsing that is in place does what is needed but I am open to suggestions.