Fail to uninstall plugins because uninstall.sh is executed unconditionnaly

Detailed bug description:

Because the prerun hook doesn't check if the plugin ships with an uninstall.sh script, the removal of plugins can fail.

Steps to reproduce:

1. Build the fuel-plugin-elasticsearch-kibana plugin from the master branch.
2. Install the plugin on the Fuel master node.
3. Remove the plugin from the Fuel master node.

Expected results:

Success.

Actual result:

Running transaction
/var/tmp/rpm-tmp.z5qrwD: line 7: syntax error near unexpected token `fi'
/var/tmp/rpm-tmp.z5qrwD: line 7: `fi'
error: %preun(elasticsearch_kibana-0.9-0.9.0-1.noarch) scriptlet failed, exit status 2
Error in PREUN scriptlet in rpm package elasticsearch_kibana-0.9-0.9.0-1.noarch
  Verifying : elasticsearch_kibana-0.9-0.9.0-1.noarch 1/1
Failed:
  elasticsearch_kibana-0.9.noarch 0:0.9.0-1
Complete!
Shell command executed with "1" exit code: yum -y remove elasticsearch_kibana-0.9

Reproducibility:

Always.

Workaround:

For plugin developers: Add uninstall.sh script in the plugin repository.
For end-users: rpm -e --noscripts <plugin package>

Impact:

The end-user can't uninstall the plugin.

Description of the environment:
 Operation system: Ubuntu
 Versions of components: N/A
 Reference architecture: N/A
 Network model: N/A
 Related projects installed: None
Additional information:
 None