env: use dynamic memory management for DPDK 18.05.1+

With all SPDK patches in place, we can now enable dynamic
memory management for all DPDK versions >= 18.05.1

Hugepages can be still reserved with [-s|--mem-size <size>]
option at application startup, but once we use them all up,
instead of failing user allocations with -ENOMEM, we'll try
to dynamically reserve even more. This allows starting DPDK
with `--mem-size 0` and using only as many hugepages as it
was actually necessary.

Change-Id: I9e6f58ea50af2234f96a53e7a32d9e14d2df65ff
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/426828


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Chandler-Test-Pool: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>