Pass node to various Build*() functions

Pass node directly from XML to these functions

- BuildClusterImage()
- BuildImage()
- BuildProxy()
- BuildRunTests()

instead of reading the "server ID" and looking up from
eNB_IPAddress/eNB1_IPAddress/eNB2_IPAddress.

Improve the proxy build unit test by properly passing ctx information.

ci-scripts/xml_files/gnb_usrp_build.xml

@@ -41,6 +41,7 @@
 		<mode>TesteNB</mode>	
 		<class>Build_eNB</class>
 		<desc>Build gNB (USRP)</desc>
+		<node>caracal</node>
 		<Build_eNB_args>--gNB -w USRP --ninja -c -P --build-lib ldpc_cuda --build-lib ldpc_aal --cmake-opt -DASN1C_EXEC=/opt/asn1c/bin/asn1c</Build_eNB_args>
 		<forced_workspace_cleanup>True</forced_workspace_cleanup>
 	</testCase>