Add systemd RequiresMountsFor and unit custom template

[guidograzioli]

Fix #35

This change adds RequiresMountsFor= to the systemd unit when shared storage is used (activemq_shared_storage=True). A new variable activemq_shared_storage_mounted (default True), allows to avoid this behaviour (ie. the shared file system is local, instances are colocated).

As an alternative it is also now possible to override the systemd configuration with a custom template, to be passed-in via the new variable activemq_service_override_template (default: ''). The file pointed by the filename should be made available in the ansible playbook template lookup directories.

Variable	Description	Default
activemq_shared_storage_mounted	Whether the systemd unit must require a mounted path (only when using shared storage)	True
activemq_service_override_template	Filename of custom systemd unit template to be deployed	``