alertmanager: inclusion in the chart

Description of the change:
- Added option to enable alertmanager.
- If option is selected then alertmanager is deployed.
- Can be deployed with or without an ingress.
- Can be deployed in ha or not ha mode.

Impact:
- No existing value is modified and alertmanager comes disabled by
default.

Testing:
- Installed locally using helm install and custom values.
- Enabled the alertmanager with ingress and connected to it.
- Created a prometheus rule firing and saw it in alertmanager.

JIRA Refs:
- MONIT-4063