acl-plugin: optimize hash memory usage + fix the startup config parsing for memory...

acl-plugin: optimize hash memory usage + fix the startup config parsing for memory sizes [VPP-1502]

In a couple of places vec_add1()-style was repeatedly called in a loop for
smallish vectors where the number of additions was known in advance.
With a test with large number of ACEs these numbers contribute to heap
fragmentation noticeably.

Minimize the number of allocations by preallocating the known size and
then resetting the length accordingly, and then calling vec_add1()

Also unify the parsing of the memory-related startup config parameters.

Change-Id: If8fba344eb1dee8f865ffe7b396ca3b6bd9dc1d0
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>