Update Maven exec plugin version to 1.3.2

Update the exec plugin version to 1.3.2. This to allow building the channel-manager addon on Windows without modifying the NodeJS distribution.
The exec plugin with the fix was released after Hippo 7.9 GA
The version is defined in the project pom.

If it is not desired to update the project pom for the maintenance branch the version can be overridden in the addon-channel-manager frontend module:
<maven.plugin.exec.version>1.3.2</maven.plugin.exec.version>

The README file of the channel manager must be updated to remove the problem description.