ofp-actions: Switch away from odd use of "q" in "enqueue" action format.

The formatting of the "enqueue" action uses a "q" to separate the port
number from the queue number, as in "enqueue:123q456".  This is different
from every other action.  This commit improves the situation by:

    * Switching the formatting to use a colon (e.g. "enqueue:123:456"),
      which is a little less odd-looking but still accepted by older
      versions of Open vSwitch.

    * Improving the parser to accept "enqueue(123,456)" also.

Signed-off-by: Ben Pfaff <blp@nicira.com>