Add input plugin for Fail2ban (#2875)

plugins/inputs/fail2ban/README.md

@@ -0,0 +1,60 @@
+# Fail2ban Plugin
+
+The fail2ban plugin gathers counts of failed and banned ip addresses from fail2ban.
+
+This plugin run fail2ban-client command, and fail2ban-client require root access.
+You have to grant telegraf to run fail2ban-client:
+
+- Run telegraf as root. (deprecate)
+- Configure sudo to grant telegraf to fail2ban-client.
+
+### Using sudo
+
+You may edit your sudo configuration with the following:
+
+``` sudo
+telegraf ALL=(root) NOPASSWD: /usr/bin/fail2ban-client status *
+```
+
+### Configuration:
+
+``` toml
+# Read metrics from fail2ban.
+[[inputs.fail2ban]]
+  ## fail2ban-client require root access.
+  ## Setting 'use_sudo' to true will make use of sudo to run fail2ban-client.
+  ## Users must configure sudo to allow telegraf user to run fail2ban-client with no password.
+  ## This plugin run only "fail2ban-client status".
+  use_sudo = false
+```
+
+### Measurements & Fields:
+
+- fail2ban
+  - failed (integer, count)
+  - banned (integer, count)
+
+### Tags:
+
+- All measurements have the following tags:
+  - jail
+  
+### Example Output:
+
+```
+# fail2ban-client status sshd
+Status for the jail: sshd
+|- Filter
+|  |- Currently failed: 5
+|  |- Total failed:     20
+|  `- File list:        /var/log/secure
+`- Actions
+   |- Currently banned: 2
+   |- Total banned:     10
+   `- Banned IP list:   192.168.0.1 192.168.0.2
+```
+
+```
+$ ./telegraf --config telegraf.conf --input-filter fail2ban --test
+fail2ban,jail=sshd failed=5i,banned=2i 1495868667000000000
+```