This adds the `bin/shield/migrate` tool that allows migrating users and
roles from the files to the native (API-based) store.
It looks like this:
```
λ bin/shield/migrate native -U http://localhost:9200 -u test_user -p changeme -n lee,foo -r role1,role2,role3,role4,foo
starting migration of users and roles...
importing users from [/home/hinmanm/scratch/elasticsearch-2.4.0-SNAPSHOT/config/shield/users]...
found existing users: [test_user, joe3, joe2]
migrating user [lee]
{"user":{"created":true}}
no user [foo] found, skipping
importing roles from [/home/hinmanm/scratch/elasticsearch-2.4.0-SNAPSHOT/config/shield/roles.yml]...
found existing roles: [marvel_user, role_query_fields, admin_role, role3, admin, remote_marvel_agent, power_user, role_new_format_name_array, role_run_as, logstash, role_fields, role_run_as1, role_new_format, kibana4_server, user, transport_client, role1.ab, role_query]
migrating role [role1]
{"role":{"created":true}}
migrating role [role2]
{"role":{"created":true}}
role [role3] already exists, skipping
migrating role [role4]
failed to migrate role [role4] with body: {"indices":[{"names":["idx2"]},{"names":["idx2"]},{"names":["idx1"]}]}
java.io.IOException: {"error":{"root_cause":[{"type":"parse_exception","reason":"failed to parse indices privileges for role [role4]. missing required [privileges] field"}],"type":"parse_exception","reason":"failed to parse indices privileges for role [role4]. missing required [privileges] field"},"status":400}
at org.elasticsearch.shield.authc.esusers.tool.ESNativeRealmMigrateTool$MigrateUserOrRoles.postURL(ESNativeRealmMigrateTool.java:206)
at org.elasticsearch.shield.authc.esusers.tool.ESNativeRealmMigrateTool$MigrateUserOrRoles.importRoles(ESNativeRealmMigrateTool.java:389)
at org.elasticsearch.shield.authc.esusers.tool.ESNativeRealmMigrateTool$MigrateUserOrRoles.execute(ESNativeRealmMigrateTool.java:171)
at org.elasticsearch.common.cli.CliTool.execute(CliTool.java:153)
at org.elasticsearch.shield.authc.esusers.tool.ESNativeRealmMigrateTool.main(ESNativeRealmMigrateTool.java:91)
Caused by: java.io.IOException: Server returned HTTP response code: 400 for URL: http://localhost:9200/_shield/role/role4
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1840)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1441)
at org.elasticsearch.shield.authc.esusers.tool.ESNativeRealmMigrateTool$MigrateUserOrRoles.postURL(ESNativeRealmMigrateTool.java:192)
... 4 more
no role [foo] found, skipping
users and roles imported.
```
Original commit: elastic/x-pack-elasticsearch@3ce47c0ffd
Lee Hinman