pull fails with azure SAS connection strings

[AmitAronovitch]

Azure SAS connection strings can be used to allow read-only access to specific container, which is useful in the context of CI pipelines and automation.

However, when using such limited-credentials connection string, dvc pull abends with the error below.

Some digging reveals that this is because remote/azure.py always tries to create the bucket (which already exists). If we check for existence before trying to create - this should work...
(will send PR)

[#########                     ] 30% Collecting informationClient-Request-ID=19129d6a-5393-11e9-80ab-5800e34ea0d9 Retry policy did not allow for a retry: Server-Timestamp=Sun, 31 Mar 2019 08:58:06 GMT, Server-Request-ID=c72fbeda-501e-0089-3c9f-e78298000000, HTTP status code=403, Exception=This request is not authorized to perform this operation. ErrorCode: AuthorizationFailure<?xml version="1.0" encoding="utf-8"?><Error><Code>AuthorizationFailure</Code><Message>This request is not authorized to perform this operation.RequestId:c72fbeda-501e-0089-3c9f-e78298000000Time:2019-03-31T08:58:06.2059267Z</Message></Error>.

<font color="#CC0000">Error</font>: failed to pull data from the cloud - This request is not authorized to perform this operation. ErrorCode: AuthorizationFailure
&lt;?xml version=&quot;1.0&quot; encoding=&quot;utf-8&quot;?&gt;&lt;Error&gt;&lt;Code&gt;AuthorizationFailure&lt;/Code&gt;&lt;Message&gt;This request is not authorized to perform this operation.
RequestId:c72fbeda-501e-0089-3c9f-e78298000000
Time:2019-03-31T08:58:06.2059267Z&lt;/Message&gt;&lt;/Error&gt;

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Please provide information about your setup
DVC version: 0.29.0+220b4b.mod
linux x86_64 py3.6 pip