Managing CA directory with known certificates

[PedroMSantosD]

When creating an icinga master, but instead of letting icinga create its ca certificates, force the use of the certificates via hieradata, the ca.crt and ca.key can not be created unless the ${ca_dir} is created.
This PR allows declaring the ${ca_dir} on the ca module; so you can choose

1. Not to set ca certificates, hence letting the module create new the new ca or
2. Set ca certificates, hence creating the ca with the content passed via hiera to the module
   icinga2::pki::ca::ca_cert
   icinga2::pki::ca::ca_key

[lbetz]

file { $ca_dir:
  ensure => directory,
}

is enough, quoting isn't needed and the dependencies are redundant, because puppet has an implicit dependency between directories and their associated files.