Add aarch64 custom config

Add aarch64 custom config

In order to run functest on an aarch64 POD, some custom configuration
is needed. Up until now that custom configuration was put on the arm
jenkins slave. With this commit it is integrated in functest project.

1. In order to identify if the POD is aarch64 or x86 function
   get_arch() from https://gerrit.opnfv.org/gerrit/#/c/29099
   is used

2. Changes to config_functest.yaml are made via a new patch file
   config_aarch64_patch.yaml which is applied for all scenarios.
   Function patch_config_file() is modified to be more generic. It
   now receives a file as parameter and merges any type of patch file

3. The aarch64 cirros images are downloaded into the data directory.
   Functest can be run from both an aarch64 or a x86 jenkins slave
   in order to test an aarch64 POD. So the images are downloded in both
   dockerfiles

4. Other custom config needs to be added to /etc/rally/rally.conf
   in the docker. The custom config is set in rally_aarch64_patch.conf
   and inserted in the needed file before the rally environment is
   set up

JIRA: ARMBAND-60

Change-Id: I8e460cdf02d297c259cbbd9c05e7983367fd2bb9
Signed-off-by: Cristina Pauna <cristina.pauna@enea.com>