travis: support versioning of DPDK and options to specify git repo and tag

travis: support versioning of DPDK and options to specify git repo and tag

Use DPDK_VER to specify the version of DPDK, and DPDK_GIT to specify
the URL of the DPDK git repository.

Both flags require "DPDK=1" and "--with-dpdk" shall no longer be
specified as part of "OPTS" environment.

Example uses:

- skip both flags to use v1.7.1 tarball from dpdk.org as before:
    KERNEL=3.17.4 DPDK=1

- specify DPDK_VER only to use a version of tarball from dpdk.org:
    KERNEL=3.17.4 DPDK=1 DPDK_VER=1.7.1

- specify both DPDK_VER and DPDK_GIT to check out the tag "v$DPDK_VER"
  from the git repository "$DPDK_GIT":
    KERNEL=3.18 DPDK=1 DPDK_VER=1.7.1sp1 DPDK_GIT=https://github.com/shushen/dpdk

Signed-off-by: Shu Shen <shu.shen@radisys.com>
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>