Some unit tests fixes

[meaksh]

What does this PR do?

This PR fixes some unit tests and prevents errors in some cases:

In cases where /etc/fstab does not exists:

unit/modules/mount_test.py::MountTestCase::test_rm_fstab FAILED
salt/modules/mount.py:333: CommandExecutionError: Couldn't read from /etc/fstab: [Errno 2] No such file or directory: '/etc/fstab'
...
tests/unit/states/file_test.py::FileTestCase::test_comment FAILED
tests/unit/states/file_test.py:956: AssertionError: {'comment': '/etc/fstab: file not found', 'changes': {[36 chars]alse} != {'comment': 'Pattern already commented', 'name': '/etc[34 chars]True}
tests/unit/states/file_test.py:1015: AssertionError: {'comment': '/etc/fstab: file not found', 'changes': {[36 chars]alse} != {'comment': 'Pattern already uncommented', 'name': '/e[36 chars]True}

Sometimes HAS_BOTO wrongly responses True even if boto module does not exists. I think it's because from __future__ import absolute_import causes that when doing import boto it does not raise ImportError as it imports utils/boto.py:

ERROR collecting unit/modules/boto_secgroup_test.py
salt-2015.8.7/tests/unit/modules/boto_secgroup_test.py:96: in <module>
    @skipIf(_has_required_boto() is False, 'The boto module must be greater than'
salt-2015.8.7/tests/unit/modules/boto_secgroup_test.py:87: in _has_required_boto
    elif LooseVersion(boto.__version__) < LooseVersion(required_boto_version):
E   AttributeError: 'module' object has no attribute '__version__'

This changes should be also included in 2016.3 and develop
Thanks!

[rallytime]

Hrm. Something is off on this test run. I'll restart it.

[rallytime]

Go Go Jenkins!

[rallytime]

Thanks @meaksh