Fix VPP vCPE VNF vGW image build issue

Fix VPP vCPE VNF vGW image build issue

The honeycomb software of VPP vCPE vGW image can not built.
The builds were dependent on snapshot artifacts.
These artifacts have now disappeared from upstream fd.io nexus.

vGW heat template files i.e. base_vcpe_vgw.env and base_vcpe_vgw.yaml are modified.
Now heat templates work with release tag of vpp and hc2vpp repositories.
v_gw_install.sh is modified accordingly.

Issue-ID: INT-525

Change-Id: I61d9d313ff619729b54cbda959d5daeec557c5b6
Signed-off-by: Praneta Paithankar <praneta.paithankar@intel.com>