Make sure we install the latest requirements

This commit switches the api server from installing the requirements
file with pip install, via the ::python::requirements class, to using
pip install -U explicitly via an exec. We need to use -U to ensure
we're pulling in the latest version of subunit2sql that's valid,
otherwise the version won't be in sync with our database which can
potentially cause issues.

Change-Id: I6797bcf82cc82a8a49441f9abb90bbc95fbd365b

manifests/api.pp

@@ -33,17 +33,17 @@ class openstack_health::api(
     require => Class['::python'],
   }
 
-  ::python::requirements { "${source_dir}/requirements.txt":
-    virtualenv => $virtualenv_dir,
-    require    => Python::Virtualenv[$virtualenv_dir],
-    subscribe  => Vcsrepo[$source_dir],
+  exec { 'requirements':
+    command     => "${virtualenv_dir}/bin/pip install -U -r ${source_dir}/requirements.txt",
+    require     => Python::Virtualenv[$virtualenv_dir],
+    subscribe   => Vcsrepo[$source_dir],
+    refreshonly => true,
   }
 
   exec { 'package-application':
     command     => "${virtualenv_dir}/bin/pip install -e ${source_dir}",
-    require     => Python::Requirements["${source_dir}/requirements.txt"],
     refreshonly => true,
-    subscribe   => Vcsrepo[$source_dir],
+    subscribe   => Exec['requirements'],
   }
 
   file { '/etc/openstack-health.conf':