Write sudo nano /usr/share/tomcat8-admin/manager/WEB-INF/web.xml
with
<servlet>
<servlet-name>Manager</servlet-name>
<servlet-class>org.apache.catalina.manager.ManagerServlet</servlet-class>
<init-param>
  <param-name>debug</param-name>
  <param-value>2</param-value>
</init-param>
</servlet>
<servlet>
<servlet-name>HTMLManager</servlet-name>
<servlet-class>org.apache.catalina.manager.HTMLManagerServlet</servlet-class>
<init-param>
  <param-name>debug</param-name>
  <param-value>2</param-value>
  </init-param>
     <!-- Uncomment this to show proxy sessions from the Backup manager or a
     StoreManager in the sessions list for an application
 <init-param>
  <param-name>showProxySessions</param-name>
  <param-value>true</param-value>
 </init-param>
-->
  <multipart-config>
  <!-- 50MB max -->
  <max-file-size>102428800</max-file-size>
  <max-request-size>102428800</max-request-size>
  <file-size-threshold>0</file-size-threshold>
  </multipart-config>
 </servlet>
 <servlet>
 <servlet-name>Status</servlet-name>
  <servlet-class>org.apache.catalina.manager.StatusManagerServlet</servlet-class>
<init-param>
  <param-name>debug</param-name>
  <param-value>0</param-value>
</init-param>
Restart Service 
sudo /etc/init.d/tomcat8 restart
Deploy geoserver.war again