Add documentation for security CLI tools

[elfisher]

Add documentation to cover the functionality of the security CLI tools.

[gatis]

May need to add this or similar example somewhere in visible place (reloading security config after you have changed default passwords in /usr/share/elasticsearch/plugins/opendistro_security/securityconfig/internal_users.yml.
Took me too many trials and errors to call it correctly.

/usr/share/elasticsearch/plugins/opendistro_security/tools/securityadmin.sh -cacert /etc/elasticsearch/root-ca.pem -cert /etc/elasticsearch/kirk.pem -key /etc/elasticsearch/kirk-key.pem -cd /usr/share/elasticsearch/plugins/opendistro_security/securityconfig/

[reilee]

Looks like we can take reference from search guard doc:
https://docs.search-guard.com/latest/sgadmin
Not sure if any difference but it works perfect for me.

[aetter]

Related: opendistro-for-elasticsearch/deprecated-security-advanced-modules#3

[aetter]

Two commits: c1fad06 and 0e9120f