Strip newline from plugin-properties

PR: https://github.com/iocage/iocage/pull/1195

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• [x] Explain the feature

• [x] Read [CONTRIBUTING.md](https://github.com/iocage/iocage/blob/master/CONTRIBUTING.md)

Fixes #1163

Fixes the regression reported as per #1163. Both the Zoneminder and MineOS community plugins would like to leverage this functionality. Can this please be ported to the FreeNAS/TrueNAS 12 branch?

The issue manifests itself as follows:
1) In `iocage fetch`, the Admin Portal has linebreak in the post-install notes
```
➜ ~ sudo iocage fetch -P mineos.json -n mineos nat=1 --branch=experimental
Plugin: mineos
Official Plugin: False
Using RELEASE: 11.3-RELEASE
Using Branch: experimental
Post-install Artifact: https://github.com/jsegaert/iocage-plugin-mineos.git
These pkgs will be installed:

• sysutils/py-rdiff-backup

• sysutils/screen

• net/rsync

• devel/gmake

• devel/git-lite

• lang/python3

• www/node10

• www/npm-node10

• java/openjdk8-jre

• ftp/wget

• shells/bash

(snip)
The default user for the Admin Portal is "mcserver" with password "mcserver"
5 warnings generated.

Admin Portal:
http
://192.168.0.167:8443

Doc URL:
https://github.com/jsegaert/iocage-plugin-mineos/tree/master#iocage-plugin-mineos
```

2) `iocage list -P` shows Admin Portal with linebreak
```
➜ ~ sudo iocage list -P

-----------------------------------------------+--------------------

--------------------------------------------------------------------------------------------------------------------+

===================================================================================================================================================================================================

-----------------------------------------------+--------------------

--------------------------------------------------------------------------------------------------------------------+
```

3) Any plugin properties requested via `iocage get -P` have a linebreak, where regular jail properties do not:
```
➜ ~ sudo iocage get boot mineos
1
➜ ~ sudo iocage get -P port mineos
8443

➜ ~
```