Filter out plugins already installed

[jimzucker]

When we pass in a list of plugins, it would be efficient if the list was filtered based on what is already installed. If the team is agrees this is a good idea I can take a shot at modifying the script.

[jimzucker]

Updated to base analysis on jenkins.war, test results:
Removing intermediate container 7f545b034f1a
Step 7 : RUN /usr/local/bin/plugins.sh /usr/share/jenkins/ref/plugins.txt
---> Running in f1606faa6f4a
... skipping already installed: ant:1.2
... skipping already installed: antisamy-markup-formatter:1.1
... skipping already installed: credentials:1.18
Downloading cucumber-testresult-plugin:0.8.2
... skipping already installed: cvs:2.11
... skipping already installed: external-monitor-job:1.4
Downloading git:2.4.0
Downloading git-client:1.19.0
... skipping already installed: javadoc:1.1
... skipping already installed: junit:1.2-beta-4
... skipping already installed: ldap:1.11
... skipping already installed: mailer:1.11
... skipping already installed: matrix-auth:1.1
... skipping already installed: matrix-project:1.4.1
... skipping already installed: maven-plugin:2.7.1
... skipping already installed: pam-auth:1.1
Downloading scm-api:1.0
... skipping already installed: script-security:1.13
... skipping already installed: ssh-credentials:1.10
... skipping already installed: ssh-slaves:1.9
... skipping already installed: subversion:1.54
... skipping already installed: translation:1.10
... skipping already installed: windows-slaves:1.0
---> 8da9a950147d
Removing intermediate container f1606faa6f4a
Step 8 : ADD jobs /var/jenkins_home/jobs

[jimzucker]

@gmacario I added the use case you suggested on PR #219 when I tested I observed with the current implimentation you have restart the docker instance for the plugins to get copied from the /ref directory and restart jenkins to see them.

I see two choices(maybe you see more):

1. I can print a messages explaining you have to restart docker for the changes to take effect
2. We can install the plugins in a different manner /restart automatically..

I am thinking approach #1 is probably the way to go for right now..

Thoughts?

[jimzucker]

@gmacario - I implemented a warning the you have to restart the container and squished the commits.

[carlossg]

#219 is merged