Use absolute data-path instead of relative. (#894)

When using the data-path internally use an absolute version instead of the path instead of a relative form.  

This fixes a null pointer exception when the data-path is set to an empty string (in a TOML config using `data-path=""` or CLI using `--data-path=`) and interprets that empty string to be the current working directory.
Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

pantheon/src/main/java/tech/pegasys/pantheon/cli/PantheonCommand.java

@@ -933,7 +933,7 @@ public class PantheonCommand implements DefaultCommandValues, Runnable {
 
   private Path dataDir() {
     if (isFullInstantiation()) {
-      return standaloneCommands.dataPath;
+      return standaloneCommands.dataPath.toAbsolutePath();
     } else if (isDocker) {
       return Paths.get(DOCKER_DATADIR_LOCATION);
     } else {

pantheon/src/test/java/tech/pegasys/pantheon/cli/PantheonCommandTest.java

@@ -303,7 +303,7 @@ public class PantheonCommandTest extends CommandTestAbstract {
             .setGenesisConfig(encodeJsonGenesis(GENESIS_VALID_JSON))
             .setBootNodes(nodes)
             .build();
-    verify(mockControllerBuilder).homePath(eq(Paths.get("~/pantheondata")));
+    verify(mockControllerBuilder).homePath(eq(Paths.get("~/pantheondata").toAbsolutePath()));
     verify(mockControllerBuilder).ethNetworkConfig(eq(networkConfig));
     verify(mockControllerBuilder).syncWithOttoman(eq(false));
 
@@ -505,10 +505,11 @@ public class PantheonCommandTest extends CommandTestAbstract {
 
     verify(mockControllerBuilder).homePath(pathArgumentCaptor.capture());
     verify(mockControllerBuilder).syncWithOttoman(eq(false));
-    verify(mockControllerBuilder).nodePrivateKeyFile(eq(path.resolve("key").toFile()));
+    verify(mockControllerBuilder)
+        .nodePrivateKeyFile(eq(path.resolve("key").toAbsolutePath().toFile()));
     verify(mockControllerBuilder).build();
 
-    assertThat(pathArgumentCaptor.getValue()).isEqualByComparingTo(path);
+    assertThat(pathArgumentCaptor.getValue()).isEqualByComparingTo(path.toAbsolutePath());
 
     assertThat(commandOutput.toString()).isEmpty();
     assertThat(commandErrorOutput.toString()).isEmpty();