Add ability to use nested directories for config

[Donien]

Using icinga2_custom_config allows deploying custom configuration files. They are read in from the files directory. This change allows for the use of nested directories instead of relying on just one directory level.

Fixes #309

[mkayontour]

I tried that the file name should be appended at the path. I guess a simple {{ item.name | basename }} could already solving this issue right?

[Donien]

The basename approach works fine when we don't want to merge files from different levels within the directory hierarchy.

given:

icinga2_custom_config:
  - name: myown_command.conf
    path: zones.d/main/myown_command.conf
    order: 10
  - name: A/myown_command.conf
    path: zones.d/main/myown_command.conf
    order: 10
  - name: C/myown_command.conf
    path: zones.d/main/myown_command.conf
    order: 10
  - name: B/myown_command.conf
    path: zones.d/main/myown_command.conf
    order: 20

In this case only C and B would end up in the file within /etc/icinga2/zones.d/main/myown_command.conf.
And that's because they use different orders. "root" (no subdir) is overridden by A, which is overridden by C. B writes a new temporary file since it has different order.

Imagine A, B and C to hold different groups of hosts that should still end up in the same file. That would only work if order was set explicitly and uniquely.

The temporary files are these:

• 10_myown_command.conf (this is C)
• 20_myown_command.conf (this is B)

With the current PR's approach we get these temporary files instead:

• 10_A_myown_command.conf
• 10_C_myown_command.conf
• 10_myown_command.conf
• 20_B_myown_command.conf

It is only more complicated so that we don't lose any information without noticing (also more flexibility).

[mkayontour]

Thinks is legit, I'll approve. Documentation wise I think this is good to go without further explanation.