Missing Tempest Tests based on cover report analysis

Gerrit topic: https://review.openstack.org/#q,topic:bp/missing-tempest-tests,n,z

Addressed by: https://review.openstack.org/21346
    Implements test_flavors.test_is_public_string_variations

Addressed by: https://review.openstack.org/21456
    Implements test_flavors.test_invalid_is_public_string

Addressed by: https://review.openstack.org/21511
    Implements test_flavors.test_invalid_minRam_filter

Addressed by: https://review.openstack.org/21517
    Implements test_server_metadata.test_create_metadata_key_error

Addressed by: https://review.openstack.org/22430
    negative tests for server metadata

Addressed by: https://review.openstack.org/22663
    Implements test_update_all_metadata_field_error.

Addressed by: https://review.openstack.org/23159
    Implements test_update_all_metadata_field_not_included.

Addressed by: https://review.openstack.org/23511
    Create tests for multiple server creation

Addressed by: https://review.openstack.org/23790
    enable xml tests test_disk_config

Addressed by: https://review.openstack.org/24842
    Enable xml tests for basic server operations test Partially implements bp missing-tempest-tests

Addressed by: https://review.openstack.org/25817
    Add fixed ip client and tests

Addressed by: https://review.openstack.org/27095
    Add tests list the availability zone

Addressed by: https://review.openstack.org/26811
    Add tests list/enable/disable the service to compute

Addressed by: https://review.openstack.org/27165
    Add XML support to test_server_basic_ops

Addressed by: https://review.openstack.org/27309
    Add tests list actions on a specified instance

Addressed by: https://review.openstack.org/27318
    Add Aggregate XML client and tests.

Addressed by: https://review.openstack.org/28505
    Add tests list tenant usage

Addressed by: https://review.openstack.org/32858
    Add tests list the availability zone