[ZEPPELIN-2139] Interpreters based on scala_2.11 aren't installed correctly

### What is this PR for?
pom variables such as `${scala.version}` are not replaced to value when you do `mvn install`.
This makes leaf poms to look for this value in parent pom, which is always `2.10.5` for `${scala.version}` and it causes scala library dependency conflict. For example, zeppelin-flink_2.11 will have scala 2.10.5 as dependency.
This PR fixes this problem by using maven flatten plugin.

### What type of PR is it?
Bug Fix

### What is the Jira issue?
[ZEPPELIN-2139](https://issues.apache.org/jira/browse/ZEPPELIN-2139)

### How should this be tested?
```
$ ./dev/change_scala_version.sh 2.11
$ mvn clean install -pl 'zeppelin-server,zeppelin-zengine,zeppelin-interpreter,flink' -am -DskipRat -DskipTests -Pscala-2.11
```
Open `~/.m2/repository/org/apache/zeppelin/zeppelin-flink_2.11/0.8.0-SNAPSHOT/zeppelin-flink_2.11-0.8.0-SNAPSHOT.pom` file and see if scala related libraries dependency version is set to 2.11.7

### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update? no
* Is there breaking changes for older versions? no
* Does this needs documentation? no

Author: Mina Lee <[email protected]>

Closes apache#2059 from minahlee/ZEPPELIN-2139 and squashes the following commits:

62d852a [Mina Lee] Change <scala.version> property in parent pom file
489c843 [Mina Lee] Use maven flatten plugin to make pom.xml variables to be replaced by value
783c014 [Mina Lee] Fix indentation and add default properties to be used in flattened-pom

Author: minahlee

.gitignore

@@ -100,6 +100,9 @@ Thumbs.db
 target/
 **/target/
 
+# maven flattened pom files
+**/.flattened-pom.xml
+
 # Generated by Jekyll 
 docs/_site/
 

dev/change_scala_version.sh

@@ -34,20 +34,22 @@ if [[ ($# -ne 1) || ( $1 == "--help") ||  $1 == "-h" ]]; then
   usage
 fi
 
-TO_VERSION=$1
+TO_VERSION="$1"
 
 check_scala_version() {
   for i in ${VALID_VERSIONS[*]}; do [ $i = "$1" ] && return 0; done
   echo "Invalid Scala version: $1. Valid versions: ${VALID_VERSIONS[*]}" 1>&2
   exit 1
 }
 
-check_scala_version "$TO_VERSION"
+check_scala_version "${TO_VERSION}"
 
-if [ $TO_VERSION = "2.11" ]; then
+if [ "${TO_VERSION}" = "2.11" ]; then
   FROM_VERSION="2.10"
+  SCALA_LIB_VERSION="2.11.7"
 else
   FROM_VERSION="2.11"
+  SCALA_LIB_VERSION="2.10.5"
 fi
 
 sed_i() {
@@ -57,11 +59,17 @@ sed_i() {
 export -f sed_i
 
 BASEDIR=$(dirname $0)/..
-find "$BASEDIR" -name 'pom.xml' -not -path '*target*' -print \
-  -exec bash -c "sed_i 's/\(artifactId.*\)_'$FROM_VERSION'/\1_'$TO_VERSION'/g' {}" \;
+find "${BASEDIR}" -name 'pom.xml' -not -path '*target*' -print \
+  -exec bash -c "sed_i 's/\(artifactId.*\)_'${FROM_VERSION}'/\1_'${TO_VERSION}'/g' {}" \;
 
-# Also update <scala.binary.version> in parent POM
+# update <scala.binary.version> in parent POM
 # Match any scala binary version to ensure idempotency
-sed_i '1,/<scala\.binary\.version>[0-9]*\.[0-9]*</s/<scala\.binary\.version>[0-9]*\.[0-9]*</<scala.binary.version>'$TO_VERSION'</' \
-  "$BASEDIR/pom.xml"
+sed_i '1,/<scala\.binary\.version>[0-9]*\.[0-9]*</s/<scala\.binary\.version>[0-9]*\.[0-9]*</<scala.binary.version>'${TO_VERSION}'</' \
+  "${BASEDIR}/pom.xml"
 
+# update <scala.version> in parent POM
+# This is to make variables in leaf pom to be substituted to real value when flattened-pom is created. 
+# maven-flatten plugin doesn't take properties defined under profile even if scala-2.11/scala-2.10 is activated via -Pscala-2.11/-Pscala-2.10,
+# and use default defined properties to create flatten pom.
+sed_i '1,/<scala\.version>[0-9]*\.[0-9]*\.[0-9]*</s/<scala\.version>[0-9]*\.[0-9]*\.[0-9]*</<scala.version>'${SCALA_LIB_VERSION}'</' \
+  "${BASEDIR}/pom.xml"