Changed RPM name to include OS ver info, ex. el7

This patch changes the RPM name to include OS version information, such
as "el7" for RHEL/CentOS 7.
diff --git a/Makefile b/Makefile
index 0ba81cb..50a6de6 100644
--- a/Makefile
+++ b/Makefile
@@ -1,21 +1,22 @@
 all: build
 
-rpm-centos7:
-	@mkdir -p rpmbuild/centos7/RPMS rpmbuild/centos7/SPECS \
-	  rpmbuild/centos7/SRPMS \
-	  rpmbuild/centos7/BUILD/etc/cloud/cloud.cfg.d \
-	  rpmbuild/centos7/BUILD/usr/lib/python2.7/site-packages/cloudinit/sources
-	docker build -f Dockerfile.rpmbuild -t rpmbuild:centos7 .
+rpm-el7:
+	@rm -fr rpmbuild/el7
+	@mkdir -p rpmbuild/el7/RPMS rpmbuild/el7/SPECS \
+	  rpmbuild/el7/SRPMS \
+	  rpmbuild/el7/BUILD/etc/cloud/cloud.cfg.d \
+	  rpmbuild/el7/BUILD/usr/lib/python2.7/site-packages/cloudinit/sources
+	docker build -f Dockerfile.rpmbuild -t rpmbuild:el7 .
 	docker run --rm -it \
 		-v $$(pwd)/rpmmacros:/root/.rpmmacros:ro \
-		-v $$(pwd)/rpmbuild/centos7:/root/rpmbuild \
-		-v $$(pwd)/rpm.centos7.spec:/root/rpmbuild/SPECS/rpm.spec:ro \
+		-v $$(pwd)/rpmbuild/el7:/root/rpmbuild \
+		-v $$(pwd)/rpm.el7.spec:/root/rpmbuild/SPECS/rpm.spec:ro \
 		-v $$(pwd)/99_vmx_guestinfo.cfg:/root/rpmbuild/BUILD/etc/cloud/cloud.cfg.d/99_vmx_guestinfo.cfg:ro \
 		-v $$(pwd)/DataSourceVmxGuestinfo.py:/root/rpmbuild/BUILD/usr/lib/python2.7/site-packages/cloudinit/sources/DataSourceVmxGuestinfo.py:ro \
-		rpmbuild:centos7 \
+		rpmbuild:el7 \
 		rpmbuild -ba /root/rpmbuild/SPECS/rpm.spec
 
-rpm: rpm-centos7
+rpm: rpm-el7
 
 build: rpm